If a wizard is so smart, knowing his resources are limited, why wouldn't they devote time to learn how to defend themselves when they aren't casting one of their two spells for the day.
You may as well ask why aren't more astrophysicists also black belt martial artists with their own dojo?
Because the study of each takes time and effort. Taking the time to fully master one takes time away from mastery of the other.
(Which, BTW, is why I have
zero problem with what happens to multiclassed casters in D&D.)
I had my own personal version of this. In college, I was a bit of a gym rat. At 5'7", I could bench 300lbs, do 3 sets of 10 reps of leg presses at 700lbs, and could jump high enough to touch the iron rim of a standard basketball goal. After getting 2 graduate degrees, I can't do any of that- I spent too much time playing "Books & Buffets" as opposed to getting any serious exercise.
You may have noticed I only mention the wizard, as opposed to the cleric who also uses the same vancian system. Because they can wear armor and swing a mace. Wizards can wear a dress and get a stick, a knife ... and a crossbow.
1) Wizards study the game universe and figure out how to manipulate it. They do this without help.
Clerics, OTOH, pray to beings who already manipulate the universe like we breathe air, and ask them to help out. IOW, they cheat.
2) IMHO, the cleric of most iterations of D&D is a pastiche of a bunch of ideas hammered into one class, so kind of got the pick of the litter, as it were. It could stand some toning down, as I've pointed out many times on this website.