Probably because systemd is the way to go these days.
Check if systemd-ui
is available from your distro's repos. It isn't on debian stable, at least not under that name.
Cron might have more graphical frontends because it's older. You can still install it, and it will work and not interfere with systemd,
Ultimately though you'll never be able to reproduce the power of systemd (or cron) in a GUI. The only compromise I can think of is a GUI wrapper script around a code editor where the end user simply composes their services as desired (requiring systemd or cron knowledge), and the wrapper script then starts/enables the jobs/services/timers etc. Technically GUI.
There's also at
but i never looked into it.
PS: I'm not arguing against your intentions here, just giving you the facts. tl;dr: look for graphical frontends for one of the 3 mentioned softwares.
PPS: it doesn't have to be a GNOME app either. Can be aQt, GTK, or any graphical toolkit app really. Though some people say you should be careful with KDE apps as they tend to pull in the whole of KDE which might or might not interfere with your existing DE.