Lots of people don't care because its not their computer. If the computer breaks or slows down or whatever, its not their fault, they just get IT involved and they get a new machine. Irresponsible people are alot of the problem with security.
Exactly, the computer is essentially a "free good" like air; people get free use of the good, but feel no responsibility for maintaining the good. We would have a lot less air pollution if, for example, a refinery smoke stack vented directly into the owner's house! That would give you a pretty clear incentive to get your operation cleaned up wouldn't it?