If you look at the per-core CPU usage in Task Manager over different conditions, you’ll see that you do get heavy usage of all cores in busy scenery areas, like flying low over Manhattan or Los Angeles.
This is because a lot of work that can be parallelized well is already parallelized well – but there’s also a lot of work that must be serialized, which can bottleneck things.
There may be additional future improvements, but generally speaking it’s already doing a good job at breaking up total computational load in areas that need it (areas with lots of scenery that must be processed on the CPU)
Note that some folks have seen an improvement in main thread times from use of DX12, which promises better ability to break graphics calls to separate threads. (I don’t use DX12 though because memory management is very broken on Nvidia cards with only 8 GiB VRAM.)