Could easily go into a very long list of reasons as a long-time app/UX developer, but playing against the computer is a good starting point: on Chess.com as a beginner I was immediately able to play against a wide variety of AI's, each is given to you by their ELO strength, and they are tuned to have different personalities so you can practice against the different types. This is so much better than Lichess where you literally choose "Strength" 1-8 (no idea what that means) and it only then clarifies "Stockfish 14 Level 8"... ok? And after the game, analysis and review interfaces have so many more helpful things for understanding the game, seeing threats, etc. Maybe if I was already very experienced with Chess I'd not mind so much. But this is just one example of many I found as I tested both a couple months ago when I was starting.

On Lichess the puzzles are less well organized and explained, the Lessons interface is arcane and much less polished in terms of content overall though there are gems if you hunt and eventually figure out the UI, and online play is likewise has a lot of small things all over that make a big difference.

In retrospect UX rather than UI would have been better in my post as I agree with this mostly (and I'd love a chance to redesign either app, that'd be a joy).