Very much not speaking for the PSF here, but my personal opinion on why that wouldn't work is that Python is a global language and collecting fees on a global basis is inherently difficult - and we don't want to discriminate against people in countries where the payment infrastructure is hard to support.

PyPI has paid organization accounts now which are beginning to form a meaningful revenue stream: https://docs.pypi.org/organization-accounts/pricing-and-paym...

Plus a small fee wouldn't deter malware authors, who would likely have easy access to stolen credit cards - which would expose PyPI to the chargebacks and fraudulent transactions world as well!