Why do all soldering irons have long barrels?
A typical soldering iron has the heating coil inside of the barrel, which influences the barrel length. There are professional soldering irons1, which use RF for heating they often have a shorter barrel.

1 Metcal, for example.  Brace for price tag.
2 Review of a soldering station with teardown.  disassembled soldering iron shown around the middle of the page.