1.Goombas are most likely distant, more primitive relatives of the mushroom people that joined Bowser out of envy for their advanced cousins
2.Their ability to kill simply by making contact lies within their slow evolutionary process, wich is yet to dispel a primitive radioactive energy that the mushroom people once had long ago.This energy was deadly enough to kill a man in an instant.
3.Goombas are safe to the Koopa Troop because Koopas, Shyguys, and other goombas are immune to this energy.
4.Mario must jump to kill them because shoes and extremely thick clothing can serve as temporary protection
5.Goombas are so expendable because they rapidly reproduce with a pregnancy period of 1 week and litters of 30 to 50!
That concludes my analasis of the goomba