Because I'm gaining more utility from the money I can make selling the materia than I can make slotting it.

Game economy is set up to shift money from battle-jobs to crafters and from there to gatherers. As someone who hates both crafting and gathering and can't sell Creator Savage runs or something, even as little as 50k would be worth more to me than 12 crit on an item. The only materia I'm even willing to slot is the free T4 materia from beast tribes, because those don't even sell high. And even there...I don't really do that content anymore, so I'd have to go out of my way to get them, which I often don't feel like.