Part of why I choose mundanes as opposed to casters is, as others have said, because I personally find it to be more fun/flavorful to fight creatures with a hunk of metal than slinging spells. Sure, a wizard could mimic a fighter very closely and exceed one in fighting ability with the right buffs, but that just doesn't have the feel/flavor I'm going for.

The other reason I choose mundanes is that they can do their thing at will. Swinging a sword can be done every round for a very long time. You might lose some hp, but that's also pretty renewable (fast healing, etc). Whenever I've played a wizard/other caster in a campaign, they've run out of spells very quickly, and my dms don't usually let us do "well, it's been 5 minutes and all your spells are gone, time to rest for 8 hours". (This is why I really want to play a warlock at some point- at will abilities and a decent amount of options? heck yeah).

Different strokes for different people, I guess.