Because it encourages users to abuse something that is a free service.
Obviously in an open source / non-proprietary world there is no way of preventing someone updating on demand (manually) rather than updating periodically (as is the norm) but not exposing it on the GUI at least discourages it i.e. encourages everyone to use the protocol and the server as it is intended.
The only time I would ever want to update manually is when updating automatically has failed, and it may be better in most cases just to fix the automatic update.
But OK all of that is just my opinion and not central to your problem.
I think you may be misinterpreting that.
The ntp package and executable and the related service is deprecated - superseded by systemd-timesyncd.
NTP the protocol (or the function) is still very much alive. systemd-timesyncd doesn’t reinvent the wheel in that sense. It still uses NTP and provides that function.
OK, but when I do timedatectl
I get
NTP service: active
and when I do systemctl status systemd-timesyncd
I get active (running) in beautiful, healthy green.