Because 1) like you point out most people are not seeking in-depth self-knowledge, and 2) most people do not have in-depth self-knowledge most of the time when people read a description they totally identify with and say "oh that's me! I'm an Idealist!" there is at most only a half-truth to this and in many cases they might be dead wrong.
In that other thread I went on and on about the nature of the ego and egocentricity and how often who people think they are is not who they really are, but a persona, or even just their actual ego, but as we know the ego is still not the sum-total of a person. This is the problem with Kiersey (and to a lesser degree MBTI) is that ultimately they are only typing your persona. As @Worriedfunction points outs there is no real psychological imperative here, despite the fact that both of these bill themselves as such. Just a superficial surface evaluation, really not that much different than saying "blondes have more fun."
You identify with Kiersey's NF based on the terms in which he lays out and how you think of yourself. But are you absolutely certain that the people around you (who are capable often of seeing your shadow tendencies even if you are not) would see you as an NF? Many so-called NFPs, for example, on this site are clear SJs they just think they're NFPs because they on some level identify with the description. But they're raging about what other people expect of them, and what they feel they need to do and so forth Fe and Te oriented stuff never realizing there is a disconnect between who they think they are who they really are as Jung warned there would be.
Furthermore it is quite common for people to think of themselves or type themselves the exact opposite of who they really are. Extraverts almost as a rule mistype as introverts because they have those moments of introversion, find them special and meaningful and assume "well I feel 'myself' during these moments of introversion I must be an introvert," never realizing that 95% of the rest of the time they are incredibly oriented to the outer object. I myself have come across a bunch of people who claim to be Fi-dominants, who are really Fe-dominants (or if they are Fi-types clearly do not lead with it). Same goes for INTPs who are really something else, sometimes intellectual Feeling types (for instance, a lot of INFJs mistype as INTP precisely because they identify with Kiersey's NT temperament, think they are an intellectual, don't want to be a feeler, especially males, and come on sites like this and exclaim "I must be an NT!" They don't realize that what they identify as NT is really Ni-Ti which Kiersey does not allow to be NT.)
Just go to the What's My Type forum and see how many people come on here saying "Am I xNFP, or xNTP?" or "I'm an NF or NT I just don't know which one," and so on. They've already made up their minds about what temperament group they MUST belong to and are now just looking for information to back it up. This is not the purpose of Psychological Type (the point is to reveal things about yourself for which you were previously unaware so that you can grow not just back up an already held notion). What happens to these people is that they don't understand that they are dealing with a persona, not a type, and because life causes personas to change and develop to the point where typing them might be only good for a snapshot of the moment not a maxim of who you are, when their persona changes they're right back in the forum saying "unsure of my type, I think I have changed types" etc. It's not their fault, mind you most of us are introduced to this stuff via Kiersey and MBTI, so like being mummified there's a lot of unwrapping we have to do to be able to get past the superficialities and oversimplifications of ideas like judging/perceiving.
Personas by nature are often given. Your persona has largely been the result of the interactions of your environment and your upbringing, and the reason people identify so closely with them (and thus Kiersey type descriptions) is because personas are adopted well before even your own ego-awareness develops (later childhood). At very young ages parents are developing their children's personas telling them to say please, and thank you, and to smile and wave goodbye. Not to yell in public and so forth. Now mind you having a persona is just a critical as wearing clothes in public, but just like clothes don't really make the man (unless you are so superficial as to believe so) persona doesn't make the man. Even if we are required to wear both publicly, both for the sake of ritual and for protection.
So the interaction of the primary caregivers and the formative environment (school, church, etc) have a profound impact on how a person comes across in the world, even if it isn't their type. This is why you might often see a family where each member is typologically different, yet everyone seems to act or speak the same. In a broader sense the same thing is true with native tribes, where again you might have a very different actual personalities (say an Intuitive seer or shaman, a Thinking or Feeling Chief, and Sensation type Warriors) yet there is still cohesion in behavior within the group. The American South has a very ESJ/Inferior Fi overtone to it, despite the fact that not all southerners are ESFJ/ESTJs, you might not know it from simple interaction. The key here is that persona IS NOT TYPE. And the more people think it is, the more problems they will have (for all the reasons I listed in that other thread about being un-self-aware). We've all seen people who over-identify with their persona (and the end result is often not pretty).
See the problem with Kiersey or MBTI is that if they were up-front about their perspective and everyone understood that they were dealing with this from a persona or social roles standpoint (artisan, guardian, idealist, judger, perceiver - what role do you play in society?) then there would really no issue. But because its presented under the guise of a sort of psychology people get tripped up often believing something about themselves that may only be half or partly true. Sort of the psychological equivalent of saying "Well because you wear black you must be an artist or emo." Maybe, maybe not.
Also I should point out that there is merit to temperament science as @Eric B and others elucidate. Its just that I think Kiersey does a somewhat poor job with his presentation of it. Berens/Nardi I think are much better at bringing both a psychological imperative and a temperament imperative together without making nearly the same number of assumptions.