Aircraft are often sold between airlines, or reconfigured even with the same airline. If United ever wants to reconfigure their 737s to be higher density (which is pretty common), they might need to start using that door as a real emergency exit.
It just seems like a lot of complexity and moving parts, for a feature that's not in use.
That's exactly how it's engineered right now, though.
This same design has been used for decades and apparently without problem, so it's probably just a case of someone designed it this way originally, and nobody thought to fix what wasn't (at the time) broken.
[1] This can still be swung out for inspection / maintenance, but has no normal latching mechanism, and should be bolted in place.