You might serve from a CDN, the CDN might not log at that granularity (at least not visibly to you as an enduser), maybe you want to dedupe refreshes/navigation

You might serve from a CDN, the CDN might not log at that granularity (at least not visibly to you as an enduser), maybe you want to dedupe refreshes/navigation (sessions), maybe people behind a NAT matter to you (when they share an IP), maybe you want to filter out bot traffic (not trivial), maybe you don't want to have to deal with log rotation and backups or want to install a GUI parser for your marketing people... a whole lot of things that are difficult to implement yourself vs dropping in one 3rd-party script and calling it a day. Google Analytics or its competitors take a few minutes to set up. Recreating that whole stack to feature parity would take forever.