PEP 591: Why Final params are not allowed? #3102

(Apologies in advance if I should ask this question somewhere else)

According to PEP 591:

Final can’t be used in annotations for function arguments

Why was that decision taken? What's the rationale?

I haven't read 100% of that PEP, but I'm pretty sure my question isn't answered anywhere.

I would really love to mark params as immutable! And this would make Py more suitable for purely-functional-programming