Because fans massively over rate young players based on ignorance and sheer optimism. Whereas players like McTominay are then quickly discarded. He's not 'crap' that's just utter garbage. There's a huge grey area between not being good enough for the direction Manchester United are going and just being 'crap'.
Nothing new, this has happened for years and every couple of years there are a few players who fans obsess about getting into the first team, moan that they aren't getting more chances and they go on to have really average careers and nobody ever thinks about them again. Majority of our fans will only have seen the likes of Hannibal in YouTube videos or tiktok clips.
Realistically very few will have actually watched him over an extended period of time and formed an objective analysis of him. Same with the very long list of players that people insisted must be in the first team but have now completely forgotten about.
Fans on average are the absolute worst analysers of football and yet stomp about with such deluded arrogance about their knowledge.
Almost all of the time they don't get the chances because they just aren't good enough and I'd very much trust ETH in his opinion on this over literally any United fan. He's seen and knows just so much more, of he decides he's not up to the first team then until he goes to Barca or something and absolutely smashes it, I'll assume his judgement is alright.
Funny because once upon a time, fans will have been saying what you're saying about Hannibal but it will have been about Jesse.