Why not run portable generator off vehicle's fuel tank?

In my OP I said "I'm sure that there is a good reason---" and I'm still convinced that is true. I'm now also convinced that none of you know that reason, either! Hey, I can guess with the best of you! My point was, an Onan draws from the vehicle's gas tank. That is seemingly it's only advantage over the smaller, lighter, quieter, more fuel efficient, and MUCH cheaper Honda, which DOES have the ability to run off of a separate tank, and no, it's not a gravity feed. (and, um, we are talking gasoline here, not diesel or propane) So it seemed logical that at some time or another somebody would have figured out a way to do this unless there was a very good reason not to. And, there probably is, but that is still a mystery to me, but we got three pages of entertaining guesses, so thanks anyway! Don't worry, I won't be hacking into my fuel system--