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.

Not everything is on a server you control (e.g. my blog is on Github pages), and not everything reaches the server with caching. Also it takes less time to setup.

Not if you're behind an edge cache (which you should be)

Some edge cache providers will give you logs of all requests.

Because we only care about pages that a client viewed.