Question about magic cost for spells

This might be a dumb question so if it is, sorry in advance.

I played OG Oblivion back in the day but only for like one playthrough and I don't remember much about it. But I'm playing remastered and my first character I'm doing a battlemage (very original, I know).

I've got my Intelligence and Willpower each to 100.

When I am at spell vendors looking at the spells available for purchase, some will tell me "not enough magicka". Wtf you mean I don't have enough magicka?? I am maxed out in the attribute that determines how much magicka my character has.

Granted, I haven't messed around with enchanting, so am I meant to be enchanting pieces of gear to increase my maximum magicka, and that's how I'm supposed to have enough to be able to cast these spells?