No, you can disable (not delete or uninstall) nondeletable (system) apps on Android without adb. adb also lets you “uninstall” them for a given user (but not from the device), which makes them less visible to the user than disabled apps, but it's not clear to me that it does anything substantively different, since neither disabled nor uninstalled-for-current-user apps run.
> Why does Android force this? There is no reason, it could easily be built into the Android GUI.
The only difference I am aware of between disabling through the GUI and uninstalling for a particular user with adb is that the former allows you to reenable through the UI if you later choose to, whereas the latter removes it from the All Apps list in Settings, making it invisible from the UI and impossible to enable.
Requiring the latter to require the user to use the same tool necessary to reverse the process makes it less likely that users will accidentally do something that the same user can't reverse if they change their mind.
You don't have to resort to ADB to disable these apps for the most part; You can do it in Settings. For some apps you cannot do that in GUI - for Settings itself, for example, so the users don't shoot themselves in the foot. Unfortunately, some manufacturers abuse this and mark as undisable-able apps, that they shouldn't. Over ADB, they can't disable it, so it works for any app, including Settings.
And the issue here is pretty clearly that users have to resort to adb to disable these apps. Why does Android force this? There is no reason, it could easily be built into the Android GUI.