Even if you felt you were fundamentally right and the manager was scapegoating you, you'd just apologise so you could train and play again, after all, surely that's the dream no matter what else is going on and you wouldn't want to let your teammates down
The issue with this is that Sancho obviously believes he doesn't get a fair chance from EtH. And that means he doesn't believe he even gets the opportunity to help his teammates (or let them actively down). So why apologize, train under a manager you believe is wrongdoing you, when you can instead train with the reserves or whatever in a hopefully better climate? And on top, when EtH gets sacked you can play the victim card to come back under a new manager.
I am sure his first assumption is wrong, but when you accept that basic belief it is actually pretty sensible behaviour from Sancho.
They're not, but it's your career also. If you can't put your ego aside to prove someone wrong for a second you'll never get anywhere in life. How do you think that looks on you also? Manager calls you out, you leave because he called you out. It's now your word against his and you're the one that left so it certainly doesn't look good on you.
You can only prove someone wrong who is willing to give you the chance to do that. If you think that this person already has made his mind up, then it is useless to even try.
And if you leave what's developing into a dumpster fire after a fight with the manager responsible for it, you don't look bad but look like someone who saw it coming and wasn't listened to.