VPNs have a limited number of users and browser fingerprints trivially distinguish users among the small pool of those using a particular VPN.
Mullvad partly addresses this issue by having their own browser with a default fingerprint. Users wanting to have anonymity can use that and benefit from other users (which don't even have to use Mullvad VPN to use the browser)
The problem is that a public service is not serving the whole public.
See my comment about the highways. Or fireworks in cities that people in the countryside cannot see. Or even worse, money transfer from one region to another. Public money usually does not serve the whole public.