They have different player populations. A pool of Elo ratings only rates players relative to the other players in the population.
If you take everyone whose FIDE rating is between 2400-2600, and you create a new pool of Elo ratings for them, their new Elo ratings will range from 1400-1600. If you take everyone whose FIDE rating is between 1400-1600, and you create a new pool of ratings for them, their new Elo ratings will range from 1400-1600. And so on.
So what does that say about the relative strengths of the players between chess.com and lichess.com? That chess.com has the more elite players from lichess.com and so the lichess.com players are weaker on average than the chess.com players?
This is a common misconception and the ratings are not comparable across player pools. lichess starts you off at 1500 and so the center of the bell curve is 1500, chess.com starts you off at 1200, so that's where it balances the 50% at.
