Why do you say that? Statistically speaking he's been very consistent throughout his career.
I've never looked at his statistics, but guess he's a near double-double guy. I remember him playing very hard in his last contract year, and even holding out until he got paid big bucks, and then played for numbers, and never elevated his game, and seemed disgruntled or disinterested. Probably missing more games than he should have (ala Kyrie Irving - another overrated player who Boston was better without completely). He played 400 straight games and since he's missed a ton. He'll get numbers and in some games (I remember one against the Pistons) playing great, but again, more me-first after he got paid, which is a habit some guys have. I think he missed pre-season games with the Cavs and playing for the Canadian National Team, until Lebron tweeted something like "Pay the man!" and got his numbers while his team has lost like crazy the past few seasons. Last year oddly enough, the Cavs get Andre Drummond (even more maturity/engine problems), already had Kevin Love, but played a lot probably because of Kevin's history of injury.. The contract is exactly the one Montrez Harrell got, who is a fine young player who puts it all on the floor. Then again, Meyers Leonard got more money and barely played in the playoffs for Miami, and only got a few minutes because Bam (and others) were out.
There has been this obsession with statistics lately, and casual fans (I know many) just look at box scores, and in arguments only use numbers and awards.. There's a TON of things that don't show up on the stats sheet, and I just go by the eye test. On the ball, off the court.... on the court, and off the court.
I was going to say "Maybe going to a winning team might help", but he won a title with the Cavs, and never really improved. I'm looking now at his stats, and he actually shot under 50% from the FT line for an entire season after getting his big 80 million dollar contract, which was a lot back then in comparison to the inflation after the last CBA.