Most people just want an allocator that works reasonably well, and maintaining that expectation means not exposing too many details that you might be held accountable to. If you care deeply, there are usually alternatives.

There's nothing really stopping systems from exposing a hint for controlling this, but usually the people who might care about it don't just want hints, but actual guarantees, and then you have to consider all the users who hint incorrectly (or correctly, but for a different system/version). So, the benefit/cost ratio is low.

Integration of GC with thread scheduling was once an active area of research, but the world has mostly moved on (perhaps prematurely, but so goes).