Honestly, because they may not have the money.
Colleges and universities are businesses. They have budgets. The schools that are need blind and guarantee to fund students have major monies and donors. That’s why there are so few of those schools.
The need-aware schools tend to be on tighter budgets. They collect monies and fees from student tuitions, room and board, parking fees, merchandise, sporting events, sponsors, awards, etc. So they make some money but . . . . .
They have to take their funding and cover labor costs like professors’ salaries, health insurance, pensions, research expenses, etc. Then, they have to pay staff who work all of those offices, cafeterias, groundskeeping, dorms, security, and parking attendants. Then, they have to pay utility fees like electricity/solar, phones, computers, water, trash collection. In other words, they have to pay their bills and stay within budget.
Scholarships have to be a part of that budget.
I was in your shoes 40+ years ago.
Private schools could fund me, but the GPA, required for my funding, (living on-campus) and work study hours were stretching my parents’ limited budget. So I switched to commuting on the buses and riding my bike, which then impacted my study hours, labs, and group meetings there.
I preferred to transfer to my cheaper, public, in-state university which was more affordable and where I received more state funding.
Not everyone can afford to go to college at the age of 18. I met a lot of people who had to delay starting college until they had significant savings.
Edited to add: One of the biggest expenses for the colleges is LIABILITY INSURANCE. 18-21 year olds tend to do some really stupid things when drunk or stoned. Staff also are guilty of harassing students or doing stupid things.