Because things like wards and barriers exist in this universe. Multiple Skills have the words in their names, multiple spells throughout the franchise focus around those abilities. Blocking consuming Magicka can 1000% be taken to mean you're channeling Magicka into whatever you're blocking with to reinforce it and essentially make it like a ward.
I think it's weird that blocking can cost magicka if we want instead of stamina, but here we are.