Scott,
The issue has to do with the pressure to the carburetor. Carbs don't like to see more than a few psi when running and don't like pressure when not running. If you have ever seen someone put a
fuel pump made for EFI on an
engine with a carb, you will know that the extra pressure doesn't
work.
You are correct that most small gas engines use gravity feed but they use very little
head height so that the pressure is extremely low.