It is not clear to me whether Sheldon or Leonard have tenure. (Raj does not.) But postdocs without tenure do not get paid what tenured professors do. Postdocs are not as loaded down with teaching undergraduate classes and doing scut work in tenured professor's program but they also do not have job security. They could easily be banking large amounts of money in case they don't get contracts, grants or fellowships renewed.
In some cases, scientists end up plowing some of their own money into their labs.
On the other hand, the show has always implied that both Bernadette and Amy live better than the boys. Also, what fugitive glimpses we've had of Raj's life seems to be much nicer than the oddest couple's. (Does this come under the "girls" heading?) For that matter, money is not an issue for Bernadette and Howard. Sitcom avoidance of reality?