Help with a player that likes the idea of being a caster, but not the mechanics

I've been a DM for about 3 years, and have predominantly run one-shots and short campaigns in DnD5e and PF2e. I have a player who persistently builds primary caster based characters, but then won't do anything in combat but "I stab it with my dagger." They rarely use cantrips, and basically won't cast a leveled spell unless I...