Because colleges want representation from a wide variety of backgrounds and experiences and applicants are viewed in context of your own situation (school, community, upbringing). The process certainly skews toward affluent applicants that have a wealth of opportunities open to them. But they hold space (and possibly funds depending on the school) to select from a broader pool that bring some interesting diversity to campus.
I’d also say if there are community service organizations to be involved in, that is a big thing someone could really jump into. Stuff that is self built can show well too. Like a theater interested teen putting together a community cabaret. An athletic teen starting a running group that meets regularly and does events.