Why do principal investigators not hire grant writers? The success of a research-active professor is related to his successful proposals for securing research funds. For this purpose, he should spend something like half of his time for writing proposals, but still he misses several possible calls for proposals due to the lack of time while the current proposals are not idea.

To resolve this problem, many people have this idea to hire a grant writer to perfectly and quickly write/re-write proposals. I personally heard this from many junior and senior professors, but in practice I have not seen any example.

This plan seems reasonable (the salary of a grant writer should be less than a postdoc), but why it is not common?

Researchers pay much more money to patent attorneys to have legally ideal text, then, why not investing on the text of proposals, which can directly enhance the chance of winning?
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.