It’s cause of his status as being one of the 3 greatest players to play the game and the media coverage surrounding him.
Yes, a lot of players and HoFers end up playing their careers on many different teams, but when it comes to Lebron because he’s the best player of his generation, he gets alot of media attention whenever he changes teams and people don’t like that.
We can have a look at some of the all time greats in comparison..
Jordan – stayed with Bulls almost all his career, then came back to play on Wizards (his fans tend to ignore this part of his career)
Kareem – stayed with the Bucks 6 seasons then forced his way to LA and played there 14 seasons. Because of how long he played for Lakers people don’t tend to think about how he got there .
Wilt – played with the Warriors 6 seasons then went to the 76ers for 4 seasons then forced his way to LA to team up with West/Baylor. Wilt actually pulled a very like Lebron move when he went to LA but I don’t think people cared about this as much because well, it happened so long ago and unlike when Lebron came there, Lakers had no championshps in LA at the time and Wilt helped them get their first one so I think people never really had ill feelings towards Wilt because of that.
Magic – played with Lakers his whole career
Kobe – played with Lakers his whole career
Bird – played with Celtics his whole career
Duncan – played with Spurs his whole career
Russell – played with Celtics his whole career
So compared with all these guys, Lebron played with Cavs 7 seasons, then left Cleveland pretty much in the worst way possible – CLE fans felt that he betrayed them, The Decision was very public, and then went to Miami for 4 years and won them 2 championships and went back to CLE and helped that franchise win their first championship, then went to LA and won them another championship.
So on paper, it doesn’t seem that bad right? 20 seasons, 3 teams, won each team he went to a championship, never forced his way out through a trade, etc so I have think the reason why he’s so hated for it is because people always compare him to players who play for one team their whole career or almost their whole career (Jordan, Magic, Kobe, Bird, Duncan), people think he ‘teams up’ with other stars (although that is actually not true for the Lakers they had a terrible roster when Lebron got there), or that he ‘leaves the franchise in ruins’ (also not really true because afaik those were Finals caliber rosters which remain the same when he leaves), or that people don’t like all the media attention around him. TBH I think it’s just cause he has so many haters. Lebron extended his contract with the Lakers and is very likely to retire with them.