OBS can be configured to have dozens of sources in dozens of scenes, all potentially active only a click or two away. OBS doesn't really have any way of knowing which one of those would be important enough to be used for the filename, and then what would the program do when you switch to a different source in a different scene?
(I'm guessing that wouldn't affect you if you don't do that, but OBS really isn't intended, at least not solely, for just single window capture where the program parses information from a single privileged source like a browser window and considers THAT to be the thing that's being captured.)