I think they do this because if it's the player and the player leaves, then the problem is gone. Conversely, if it's not the player, then that means there's a bigger problem, one with the coaching, and/or scouting, and/or the FO, and that's scarier, and harder to change, and will take more time. It's also easier to just blame it on the player, than trying to figure out what the problem is and how do they fix it.
Of course I realize that, I'm just calling out those fans that think it's always the player's fault while ignoring the much bigger fault that lies with the organization. It's no coincidence that this franchise hasn't been able to find and develop a young QB into a FQB, and it's not "bad luck", it's systemic. What's a young player supposed to do when drafted into a situation like this? Try to adapt. From all reports that what Zach tried to do, and he kept his mouth shut. Where did that get him? To become the target of fan anger and frustration while the real problem was in the GM and CS. Yes, it's clear that Zach was over drafted. Maybe he had an entitled attitude - although AFIK none here were in the locker room or witnessed his interactions with his teammates so how reliable are these reports. But let's just say they're true. None of that changes the fact that it was poor work by the GM in identifying his talent correctly, and then subsequently not building the best environment for him to succeed with a competent CS and good supporting players - especially a good OL. And it was incompetent coaching that failed to design an offensive system and weekly game plans that would take advantage of whatever talent he has, and minimize his shortcomings. At best, this team's decades long failure in this regard is due to benign neglect; at worst it's professional malpractice. But it's all Zach's fault.