Why not? I had my Northstar GPS coupled to the Century III onboard my Bellanca. I could switch input to the autopilot from heading to either VOR or the GPS. While your assertion that a Baron might not be equipped with GPS is not all pervasive, it certainly is a good bet to assume it was on board and capable of providing guidance through whatever flavor of A/P that was installed. I cannot think of one good reason why I would have an IFR GPS NOT capable of coupling to the A/P. Do you have a reason this would be so? I would also think an aircraft in the Baron's class would also be equipped with a flight director.
Why not?
Simple: money.
It costs money to slave the autopilot to the GPS. Depending on your routine missions, you may not wish to spend the money. I don't. I just keep the heading bug on the course the GPS gives me, and that does the trick.
I assume you know enough about flying not to assert that not slaving the two amounts to a "safety lapse" or some such thing.