ELI5: Why can't we substitute food for pills/capsules?

If we can supplement vitamins why can't we survive by ingesting pills that supplement the rest of the nutrition we need vs. eating actual food? Is it possible?

EDIT: Just to save you the time of typing more than you need to, I used vitamins as an example, I'm more interested in the other elements we need to survive and if its possible for us to sustain life with those in pills or not.