Because it is impossible to play ETH-football with a striker that does not press and wants to be the focal point of all moves forward. Ronaldo had the lowest pressing numbers in all the top 5 leagues last season. Of the 800+ midfielders and attackers in the top 5 leagues with more than 500 minutes last season he was rock bottom of the list at pressings/90. Putting him in the 1 percentile group for pressings/90 is actually an insult to the other 12 players in the group, because he was in the 0,1 percentile, alone. It's not possible for any modern progressive team to function well with a player like that on the pitch against top opposition. Not in any scenarios, except in your dreams maybe.
Ronaldo spent the first 3/4 of the season being the most wastefull player in our team, remember when he, in a 3 months period from December to March, scored 2 open play goals from 49 shots (in 14 games) + 1 missed and 1 scored penalty. He missed sitters every other week. His only assist in that period was a bad first touch picked up by McTom as well. Ronaldo's teammates scored 1,42 goals less than expected from Ronaldo passes last season in total. 1,42! That Sancho, Rashford and Bruno missed a couple of sitters is nothing compared to that period for Ronaldo. When he at the same time contributes zero to the collective and throws his arms in the air every time he makes a run and doesn't receive the ball, the net negative will always be huge, no matter how many goals he scores. We scored 0,42 goals less and conceded 0,35 goals more per match (0,77 goals worse goaldiff per match) in the PL last season compared to the season before. Not one non-goalkeeper player improved with his presence in the club, quite the opposite. Everyone got worse. His "elite mentality" didn't rub off. We changed the way we played when he arrived and every measurable team metric got worse as soon as he arrived (chances created, chances conceded, possession, counters taken, in which areas we won possession/lost possession, etc.).
This is of course also why we have a hard time getting rid of him now. Hopefully he'll take a massive paycut and some team will bite, but I doubt very much it'll be title contenders in any of the top 5 leagues.