https://fedoraproject.org/wiki/Packaging:Systemd#Socket_activation says:

Simila to inetd, using socket activation for on-demand loading will impose a startup time penalty so we currently do not use this feature in Fedora.

and

Since Fedora currently doesn't want any services to do on-demand loading, all socket activated services must autostart.

The motivation is obsolete (if it was ever true). A full Fedora container boots in ~150 ms, so the penalty for starting a single service is negligible. Starting socket activated services on demand conserves resources.

In fact, contrary to the guidelines, there's a bunch of socket activated services already, in systmed itself: systemd-localed, systemd-machined, systemd-timedated, systemd-importd. They work just fine this way.