Because on his day he's magical.
He's the definition of a 'glass cannon'. A luxury player. He's one of the few genuinely world class footballers around, but unlike the others he breaks down constantly. So it's the risk/reward ratio - do you have him and hope he has enough 'moments' to justify his cost, or do you not bother because you can't reliably build a team around him.
If we were a top side, he'd be worth it for the X Factor. But we don't, so on balance, for me, he's not worth keeping.
There's no one to "blame" for this situation. Benitez needs to manage the club his way, it's what we've been screaming out for - direction. Ancelotti was allowed to bring in Allan and James, Benitez doesn't want the latter for how he'll set up. That's fine. It is what it is.
Thing is Tubey, while I agree with your point, the reason we can't 'afford' the luxury player is genuinely because we can't literally afford him.
Regardless of Benitez/James here, I am frustrated by that, because we're a mess and it's mostly self-enflicted. COVID has played its part, but the buck lands directly at our DoF and board.