Why do principal investigators not hire grant writers?
Most research staff and many students are paid by grants, but most grant terms do not allow for you to pay for administrative staff or proposal writing time. So the funding has to come from the universities or departments. Most of them don't have the funds to hire a grant writer, though some do. I think most departments would rather have half the funding for another professor than a grant writer. I also think your estimate of the salary of a good grant writer in the sciences is off by large margin. The good ones, employed by institutes with large enough institutional budgets make as much as full-time researchers or professors.