Because, IIRC, xscreensaver is launched on demand (idle timer, power management), and that's a terrible time to detect the extension and tell a user that they won't be able to resume their session, because their Xserver is too old.

Also, because of piecemeal releases, and remote X. You might update Xscreensaver, but not your X server or desktop environment. You might have a dedicated X terminal which can't easily have its server component updated, but you run remote sessions that have an updated Xscreensaver. (Btw, if you do this, you're pretty dedicated in 2021)