FIDE is more professional and has higher rating standards that cant be given on online sites
secondly chess.com is meant for practice and fun . where as FIDE is for Chess as a sport where people compete with each other
on chess.com people may cheat , would be using offline recourses , or maybe getting their ratings boosted by other players
FIDE cannot take risk like this they want more real ratings to make sure they get the right rating and they know who is a real player and who is a person behind computer doing other stuffs
I feel like it would make a lot of peoples' lives better and easier if your chess.com rating reflected your FIDE rating, which would allow for people who either can't go to FIDE tournaments or don't want to to get FIDE recognized titles from their house.