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Why not use something else to ban users in this case? For example the user agent, browser window resolution, installed fonts, etc.

Those can all be changed. A determined spammer could make a bot that automatically changes them.

Whereas an IP can't be trivially masked?

Most free proxies are already blocked on wikipedia. Finding more once yours have been block I don't think is very easy.

Browser configurations can be easily generated by a computer and you never run out.

Does Wikipedia block editing from AWS IPs?

I assume a browser fingerprint of the right granularity is hard to change for someone who would vandalize Wikipedia without a proxy.

Because vandals never use different browsers? ;-)