Pilots are trained to safely fly the aircraft in case of co-pilot incapacitation:
"A commercial aircraft can be landed with one pilot at the control and Delta pilots are fully trained to do so if the situation were to occur." (Delta representative)
So why would a pilot in such a situation elect to ask over the PA system "are there any non-revenue pilots on board, please ring your call button" and then land the plane together with this unknown person (a military pilot in this case) ?
This happened on United Airlines Flight 1637. Sure, if it turns out right (which it did in this case), it's certainly pleasant to have a helping hand. But there are all sort of things which could go wrong (“At the beginning, I interrupted her flow of operations, but we figured everything out extremely quickly."). Personally, I'd much rather prefer not being interrupted and distracted when I have to concentrate in this critical moment. Why take any risk at all when one pilot should be able to safely land alone?