Exactly. The way I see it, there are a few possibilities:
They like the wizard aesthetic but prefer melee combat. Seems unlikely since you'd think they would have picked a different class and flavored them as an inept magic user.
They want to play this funny character idea at the expense of their party. Dick move but also seems far-fetched considering how good they are as a player otherwise.
They want to be a useful spellcaster but they haven't gotten a grasp of the rules quite yet. I'm really leaning towards this being the case, especially considering this:
They rarely use cantrips, and basically won't cast a leveled spell unless I suggest it immediately before their turn.
So they want to use spells but they're unsure of the best time and place to use them? As a DM I'm thinking: ok, lets get you some spell cards so you have descriptions handy, maybe share your spell list with other players so they can help you decide what to use and I'll do my best to remind you of which spells work best in different scenarios!