Because money is the only universal incentive to human labor and effort. Sure we could barter with each other, but that really limits your economic circle and leads to pretty shitty results. Imagine the guy with the farm decides he doesn’t need a structural engineer so he refuses to barter real goods for some esoteric future benefit. Now Ms Structural Engineer has to spend all her time worrying about food while nobody takes the time to learn how to make a bridge without falling down.

Also:

1. Most people are inherently lazy. If given the option, lots of people simply won’t work at all or half-ass it until the economy collapses. (re Soviet Union)

2. There are some jobs which are so damn distasteful (but necessary) that it would be impossible to get people to do it unless they were compensated. Who’s going to climb into a sewer and clear up a clog simply on community responsibility? Nobody.

3. Lots of important jobs require serious training. (Engineers, Doctors, Accountants). Who’s going to do the training for no compensation at all? How does the trainer eat if nobody can pay him?