Kenja no Mago: Wise Man's Grandchild

[KrayZ33]

The one explanation I can come up with is because he needs to visualize things to cast it.

Maybe the object acts as some kind of anchor for that

And since magic power is involved, using generic anime magic logic, it might not drain as much spellpower as large scale spells.

I have to say though, this is the least interesting Isekai I've seen yet.
The first episode was "okay" (generic etc. but you'd think you can make something out of the setting) and then it got strictly worse with each new one... and now it's already at a point where it can't get much worse.