RowJimmy Think it's about being realistic: you can't expect 22 year olds, even under ideal circumstances, to start at one of the world's biggest clubs that also plays in the toughest league in the world. And circumstances haven't been ideal at Manchester United for decades.
Höjlund looked okay on the onset in his United career too: he scored 16 goals in his first season. But Höjlund is a vertical channel runner who finds space in-behind, not a holdup player, and he couldn't lead the line for United; even less so a completely dysfunctional United side.
Sesko is similar in that he's a second striker and not an actual striker. He has no fight in him for one thing; he's too slow in the decisive actions for another. Premier League was the wrong move for him and United exacerbate that to some degree, but he's not going to go somewhere else and suddenly be brilliant up front. He might find a manager who uses his strengths better, like Höjlund did with Conte, but he is what he is right now.
Players can change and develop of course, but you pay for that with points on the pitch. Back when football still had a dimension of on-the-field logic to it, this was the idea behind big transfer fees: you pay to avoid having to do that development yourself.