Because it's available in the cheap models, including a model (the Mac Mini) that costs about as much as the AMD 5850U cpu itself.
Also, I see the M1 as having 1733 (single) and 7652 (multi) GeekBench score, whereas the AMD Ryzen 7 Pro 5850U has 1414 (single) and 8140 (multi), so the M1 is quite faster in single core than it (22%) and close in multi-core (the AMD being 6% faster).
So what gives, and how does that fits with the numbers given by the grandparent?
Because so far Apple has only released entry level products with the M1. It’s pretty obvious that they will release higher end chips for higher end products later this year.