Sometimes MacOS and iOS apps don’t work properly. While people should know to contact the developer, I bet quite a few of them contact Apple. I know some do, from conversations with ‘geniuses’ that I’ve overheard when picking up items at Apple Stores. Opening up a system to user-driven modifications is going to lead to increased support costs.
Developing and implementing a ‘simple scripting language’ will also incur costs. Making it secure enough to prevent unintentional poorly executed code might be possible, but that adds cost and makes the language less simple.
But what about intentional, malicious exploits? cameras are used in sensitive settings, e.g. journalists in hostile environments (where governments have the desire to control reporting), police and private investigations, etc. even devices touted as closed and secure (iPhones) can be tracked and hacked by nefarious governments. A device with sensitive information, Wi-Fi, GPS, and an open scripting language is a gaping security hole. Fighting the necessary ongoing battle against malicious exploits is neither cheap nor simple.
The fact that you think that this would be easy suggests you haven’t really thought through the ramifications.
As for your earlier argument:
I suspect such a feature was not added because not adding it allows Canon to make a profit on selling items such as a remote shutter release with a timer, rather than letting users write a script that instructs the camera to take a photo every X seconds.
Are you aware that Canon has included an interval timer on models starting with the 80D?