Nothing really, apart from the fact that most people who purchase bitcoin, do it on some centralized exchange and almost all of them require KYC. So unlike the bad title OP chose, the centralized service has KYC not bitcoin.
The importance of it comes down to how much the user believes in financial sovereignty. For most people, it is all about making some profit in which case KYC is not as big deal (which is why CEX is still popular!) but for anybody who seeks financial sovereignty, any form of surveillance (that includes KYC) is in complete contradiction.
You are absolutely right that centralized exchanges that ask for personal information but Bitcoin itself does not do that.
If anyone mainly trying to make profit then giving information for KYC usually is not big deal which is why these websites are still so popular. But if any truly want to be completely independent with their money and not have anyone watching what they do then any kind of tracking goes against that idea. It shows basic conflict in world of cryptocurrencies it is trade off between making it easy for people to get crypto through CEXs and original idea of Bitcoin being private and not controlled by anyone.