[nickvincent]

In practice, I believe the reason is historical: that's how Wikipedia started and it hasn't changed. But there's pretty compelling evidence that it provides Wikipedia some unique benefits relative to an account-locked alternative:

Hill, B. M. and Shaw, A. (2021) ‘The Hidden Costs of Requiring Accounts: Quasi-Experimental Evidence From Peer Production’, Communication Research, 48(6), pp. 771–795. doi: 10.1177/0093650220910345.