Folks would rather have every future time sliced so that other tasks get some CPU time in a ~fair way (after all, there is no concept of task priority in most runtime).
But you're right: it isn't required, and you could sprinkle every loop of your code with yielding statements. But knowing when to yield is impossible for a future. If nothing else is running, it shouldn't yield. If many things are running but the problem space of the future is small, it probably shouldn't yield either, etc.
You simply do not have the necessary information in your future to make an informed decision. You need some global entity to keep track of everything and either yield for you or tell you when you should yield. Tokio does the former, Glommio does the latter.
It gets even more complex when you add IO into the mix because you need to submit IO requests in a way that saturates the network/nvme drives/whatever. So if a future submits an IO request, it's probably advantageous to yield immediately afterward so that other futures may do so as well. That's how you maximize throughput. But as I said, that's a very hard problem to solve.
