Because it is so often capitalized, lowercasing it looks like a mistake, it looks exactly like an acronym, Elo was not nearly as famous for anything else, 'ELO' exists as a real acronym for many other things (eg. Electric Light Orchestra), and you can even easily define pseudo-definitions for it as a backronym - 'expected logistic odds', just off the top of my head.

I host a copy of his book and I still struggle every time to not type 'ELO' instead.

Most likely reason is because it doesn't matter, "Elo" stopped being a person long ago, similar to many named streets, cities, states, etc. throughout the world

ELO is just as accurate as Elo when referring to the rating system.

And why do they use Elo to refer to rating on chess.com and lichess? [They use Glicko/Glicko2].

They aren't misspelling, they are capitalizing on the name.