Well, the reason it hasn't happened yet is basically just that undo is a really bloody complicated feature to add, if you haven't planned an application for it from the outset.
Basically, most application APIs, including Qt, offer something like an "undo stack" that is used for supporting undo operations within applications. To use the undo stack, you need to wrap every change you make to the application's state in an "undo unit", and send that unit through the undo API to be processed. The "undo unit" also needs to provide instructions for how to reverse the state change it describes. Then, if the user actually executes an undo operation, the most recent undo unit can be pulled off the stack, and have its reversal operation applied. (For the curious, Qt's documentation of the API involved is
here.)
Now, OBS Studio wasn't built with undo behavior in mind. That means that any time you take an action in the UI, that action is executed directly on the data; it isn't wrapped into an undo unit and stored in the stack. So to implement undoing, someone would need to go through every single action you can possibly take in the OBS Studio UI, convert it into an undo unit, and write some code to reverse the action. It would be a pretty sprawling change that would affect nearly everything in OBS Studio, and would require quite a bit of testing to make sure that nothing gets broken in the process. So, understandably, no one has yet chosen to commit the time it would take to implement this.
It's unfortunate, because it's a pretty glaring missing feature, and I definitely wish it had been built into the structure of OBS Studio from the beginning. But adding it now would be a pretty big project for someone.
