There are technical limitations, sure, (getting an AI to parse a screen and interact with it via mouse and keyboard is harder than it sounds - and it sounds hard to start with) but the main limitation is still economical. Does it really make sense to train a multi-billion-parameter AI to click buttons, if you could instead just make an API call?

There's an intersection between "high accuracy" and "low cost" that AI has not quite reached yet for this sort of task, when compared to simpler and cheaper alternatives.