Admin powers should be granted automatically to users who have been around a while, basically.
Not everyone that's been around a while is suitable to become an admin. We generally try and avoid banning people if it can be helped, if not banning someone would result in them becoming an admin, we would end up with no choice but to ban them.
Also, it's necessary for admins not only to be trustworthy, but to be trusted. We have enough problems with that as it is, making everyone that's been around for 6 months (or whatever) an admin would reduce that trust even more.