Plenty of applications are single threaded and it's cheaper to spend thousands on a super fast CPU to run it as fast as possible than spend tens of thousands on a programmer to rewrite the code to be more parallel.
And like you say, plenty of times it is infeasible to rewrite the code because its third party code for which you don't have the source or the rights.
And like you say, plenty of times it is infeasible to rewrite the code because its third party code for which you don't have the source or the rights.