The difference is that Hojlund doesn't have anything to justify him staying.
I watched a video showing that he has in the PL
- One of the lowest touches in the box
- One of the lowest shots taken
- Low progressive passes
- Low defensive attributes
- Low hold up play
- Of course and no goal output
It's not a coincidence many of us feel like we are playing with 10 men when Hojlund is playing up top because he really isn't doing anything.
Zirkzee on the other hand is quite good or atleast capable at all of those things except actually scoring a goal.
I think it's why Zirkzee is quite polarising. He has a variety of abilities and his skill can open up defences but at the same time he doesn't seem predatory for a player that is occupying the number 9 spot and some find him slow (I don't think he looks slow after January last season).