Because schools get to decide what your need is, not you. I’ve watched many students sit down, have a conversation with their parents (as they should), and the family crunches their numbers and determines they really can’t afford anything/much of anything for college. Turns out colleges rarely think your need is what you think your need is – and meets-full-need colleges only promise to meet what they think it is.
And, many can meet need with loans. Which, on top of a parent contribution, student contribution, and work study, often turns out to be unaffordable for families. Even at schools that promise to meet need with no loans, parent and/or student contributions are still often expected through work study and/or summer work, etc – opportunities that aren’t guaranteed, and that many low income students find they cannot afford after already having to foot the bill for travel to and from campus, personal expenses, fees, etc).