> A few single-threaded processes will use my cores just fine, thanks.
Depend how many core you have.
But the issue is that algorithms that scale linearly are not known for a lot of problems. In most case, one get a speed up of at best 3, while using two time more total CPU time (on 8 cores. Using more cores would only increase the wastage but not the speed). So the incentive is not that large.