- they use different rating systems (Elo for FIDE, Glicko-2 for LiChess and afair, Glicko/Glicko-2 for chess.com)
- ELO / Glicko rating is calculated for a _player pool_. Which means it will never correspond between FIDE-registered players and chess websites because they don't have the same player base.
It has nothing to do with being “inflated” or “unrealistic”. The difference is “by design”
Of course these things are correlated, but e.g. there have been times in the world championship when one of the players thought about a position for 20+ minutes before making a move. That's hard to do, and requires very strong visualization skills.
150 Elo on levels below 2700 FIDE is almost night and day difference.
Chess.com ratings show nothing about theoretical FIDE ratings. Comparison tables have been built with data mining and lucky guesses, there's no quality difference between Chess.com <-> FIDE and LiChess <-> FIDE.
It's just a fact that chess.com ratings are closer to 'real' ratings than Lichess are. It's not perfect but that doesn't mean it's a zero-information statement either.