They don't fit well because:
1) Airsoft gun manufacturers don't follow any sort of standards for thread form, which means all their threads are different
2) Threads are NOT positive locating mechanical interfaces. Due to the required tolerance between male and female threads, there will ALWAYS be radial slop.
3) Barrels require pretty intense tolerances during manufacture to be considered "high quality", and so anything extension to a barrel would need to be equally as high quality both in bore and in the interface between the two.
4) You'd absolutely need an air tight seal between the two barrels, otherwise the amount of compressed air lost at the interface would always either reduce accuracy, or make the extension useless.
Easier to just upgrade the spring to get higher fps.