Yeah, I am with you on this. It isn't even the case that Lichess UI is simple because it lacks advanced features, it just straight up feels more intuitive and way more pleasant to interact with. Chess.com UI reminds me of overcluttered websites from late 2000s/early 2010s.
Pure anecdata, but I only know one person irl who prefers chess.com, and I never managed to get a straight answer from him as to why, other than "i just play there and like it more, maybe i will check out Lichess at some point, idk."
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.
Worryingly, I disagree with your opinions despite your credentials. There's no accounting for taste, but then of course, all the carefully designed interfaces I don't like were made by people who thought they were great.
Some of the UI is really tiny on the smart phone version of the lichess app is my only complaint against their UI vs chess.com. As far as I'm concerned though they both could use a good designer. Neither of them are very good.
Besides that I tend to use, and pay for, chess.com more for their other features than for anything directly UI related. Their puzzles are better. I think their post game analysis is nicer to use, the way they do analysis in general is easier to use than lichess I think. They also have a lot of learning material.
As far as playing games go, I have friends who use both and therefor I play on both. It's about the same as far as I'm concerned.
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).
