What's the point in hiring assistant professors instead of more experienced professors?
For example:


Their salaries are far lower. (The discrepancy varies by department, high in the physical sciences, low in economics and business.)
In fields that receive startup packages, their startup packages are far lower.
Many schools cannot hire a proven superstar, but they can take a gamble on a bright young scholar and help her develop into a superstar
This doesn't apply in my department, but you could imagine that a disfunctional environment the senior faculty might like having junior faculty around to take advantage of: make them teach the big intro classes, do the unpleasant service work, etc.
Junior faculty often bring new bleeding-edge skills that are hard to find in senior faculty.
A range of career stages within a department creates long-term continuity.


I'm sure others can come up with a number of additional reasons.