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.
Since they don't seem to have any teaching responsibilities, and they seem to command more respect than postdoctoral scholars, I assumed they had professional faculty titles. Not sure what these would be at other universities, where I work they would be something like Specialist, Researcher, or Project Scientist. None of these are tenured positions, but if the person is valuable enough to the university they will pay to keep them around as long as possible. Of course, they would probably be encouraged to move to a tenure track professorial title.....why am I discussing the fake jobs of TV characters again?
It's the weekend, last thing I want to do is think about work!