Actually, post-docs are the cheap labor. PhD students are given a stipend and many/most have their tuition waived. The overhead of running a research department in the US is fairly high and leaves little room for high salaries[1]. Anything can be improved, and its easy to criticize from the outside.

[1] In the main-stream case, not withstanding the egregious greed that sometimes occurs at the management level. But no industry is immune from human nature.