Because GPU's can handle higher temps for longer, than a CPU can. A GPU will happily run 80C+ 24/7, but running a CPU like that will shorten it's lifespan to some extent, worsening the hotter it gets.
Besides, it's always best to keep everything the coolest that you can to make everything last. If you can afford to burn out and buy new stuff every year, them more power to you, but most people can't.
And 72.6C is the max CPU temp for most Sandy Bridge CPU's so that's where that figure comes from (although max core temp is 98C).