Zelalem is 20 (not that that's old) and has a grand total of 1 career goal at the senior club level.
He's a 20 year old midfielder, not a 25 year old striker. You understand that, right?
Arsenal thinks enough of him that they signed him to a contract extension earlier this year. They obviously think more highly of him than you do.
He's an attacking midfielder. Maybe Arsenal thinks more of him than I do but I am sure you realize that these clubs have 30-50 players on their books. Most are "lottery tickets." Maybe they pan out but in the meantime, they pay his salary by loaning him out. Arsenal has very little invested in the kid.
I hope it works out for him but I don't see it.