I think perhaps someone's DM coddles them.

Heck, four 4th level wizards, properly equipped, could take down the tarrasque. Fly + Melf's acid arrow for the win. -Lycanthromancer
Hrm, Tarrasque base hp is +594 (not including it's HD)

4th level acid arrow deals, in two rounds, total of 4d4 x 2 casts per wizard = 8d4 x 4 wizards = 16d4....so it's maxxed out at 60 damage or about 1/10th of the bonus hp a Tarrasque gets.

You've vastly overstated the power of a wizard by just assuming that at any given moment they have every single spell that exists memorized.