I have seen this question asked here at least 100 times. You know the Chinese proverb, a picture is worth a 1000 words. I finally remembered to take a picture after I hooked up to a goose neck trailer so I could try to answer this question.
On a conventional fifth wheel the pin on the trailer is welded to the trailer frame. When you hook up your truck to the trailer you are then pulling directly on the trailer frame.
If you use a gooseneck adaptor your truck is now pulling the trailer from the end of this adaptor. The adaptor is not braced as in the picture below.
Think of the adaptor as being a “cheater” on a wrench. With a cheater on a wrench you can apply greater torque. This increased torque can break the welds on the trailer pin.
In the photo below notice how the vertical pin on the goose hitch is braced.