Personally, I think that has to do with the fact that Fi is also never appreciated. Or so it seems. All people do is whine about it. Especially ENTPs, no offense. So at some point you go 'yeah yeah, we get it, you don't however, clearly!' and become defensive.
As said before, maybe I'm biased, but Fi seems to be one of the least appreciated functions around. It's also not practical, doesn't deserve merit by being particularly useful to others, and when it is actually useful, it doesn't get acknowledged or it gets treated as nice 'luxury' to have (thinking art, music, hugging, fantasy books, lifting someone elses spirit, etc). It doesn't feel like it provides for the necessities in life, it doesn't help you in the real world, or so it seems, therefore it's not valued. On the contrary, it gets more press for being a burden and in the way of 'valuable' things than anything else. I get it, Fi is hard to pinpoint, to quantify, to put to use, and to even fully appreciate becoz it's so ellusive and highly individual, and therefore hard to grasp for those not employing it as much. It's not our fault that you have trouble seeing its merit though. And although some of the comments of how you perceive Fi definitely apply to immature Fi, some are so far off becoz of lack of understanding, it just makes you wanna roll your eyes, coz you know that explaining it wouldn't matter as you cannot prove your stance to them anyway. It's the proverbial dog who's seen a rainbow and who tries to convince other dogs it exists-thing all over again. Can you honestly blame us for our reaction?