Because they can't feel good about themselves under any circumstances. That's the thing.
In order to be happy about your accomplishments, you need to see yourself as a worthy individual to begin with. You learnt something new, you made some progress - great. You did a good job and you give yourself a pat on the back.
But perfectionists never feel this way. No achievement is ever good enough for them. They have this unquenchable thirst for self-validation. It's toxic and destructive, and they never get there.
Not to be confused with true passion, though. Some people are deeply passionate about a particular subject. If the stars align, they end up being those cool experts and professors. :-) You can't fake it, though. You can't force yourself into truly loving something.
That's my take on it, anyway. :-)