Because "Wait. I'll have to learn more than one subsystem! No."It is incredibly stupid to me that things like Hunter's Mark, Hex, pact boons, smites, and other such things have been, or are being, turned into spells. They're locked into the rigid mechanical and thematic structure of spells. These things should be their own distinct things, allowed the mechanical and thematic freedom to take whatever form makes sense, not beholden to working the way Wizards work. That's why they're distinct things, mechanically and thematically, and not something just any old Wizard could learn!
It's funny that we have a "When the fiction doesn't match the mechanics" thread on the page at this time.