Instead of having to support the last 5 versions of the browser you have to go back and support versions that are YEARS out of date regarding web standards and implementations, so even when Safari does add an upcoming feature you have to wait a few years before all users actually update their macOS to get these new features.
A quick example I just grabbed is AVIF support [0]. Safari has technically already added full support for this image encoding, the problem however is that you NEED to be on macOS 13 for even partial support, and full support only appeared later. If you look at the usage statistics at [0] you'll see that most Safari users are still not on the newer versions. It's even worse on iOS regarding some things, where even mundane features for Flexbox might not be available bug-free to all your users.
Rant over.
