Because that involves storing a cookie, which you're not allowed to do except as necessary to operate the site without first gaining consent. I use Plausible,

Because that involves storing a cookie, which you're not allowed to do except as necessary to operate the site without first gaining consent. I use Plausible, which works by hashing IP addresses. Honestly, I disagree with the analysis as given: the claim is that we're assigning a unique ID to each user, but it's really not doing that -- the ID changes, and more than one user may have the same ID. GDPR is a balancing act, between the rights of the people involved and the desires of data controllers to have useful data. In practice, Plausible presents (at least to my mind) a solution to my legitimate interests with a privacy impact that's suitably minimal for the level of interest I have in the statistics I collect. A cookie is arguably less privacy-preserving, as it has better specificity.