And they are that way because we have used hard-won experience earned in blood to spend hundreds of years designing unnecessary complexity and failure points out of them.
Cartridge ammunition small arms are one of the most refined and matured technologies on Earth.
The only - ONLY - consistent reason that people have attempted to add significant complexity back into them is in convoluted, ideologically-motivated attempts to make them less accessible and reliable, and that impetus has always been based on the belief that by doing so, their use will be discouraged.
Notice that nobody hawking these devices ever suggests the military or law enforcement should be mandated to use them. Just the filthy plebs.