Why? Well in the USA, it's likely because unis want your tuition dollars, so once admitted, they have more of an investment in retention of a student. Public K-12 schools get a certain amount of money per student, and a certain additional amount per special education student (nothing additional for 504 accommodation students), regardless of the level of service offered. Resources are zero-sum. More service for this student means less available for the next student. And you pay for all of those assistive tech licenses. This is also why certain supports are sometimes more available in high needs schools than in moderate need schools: as with free/reduced lunch, once your population exceeds a certain percentage of qualifying students, there are provisions for declaring the whole school qualifying, and subsidizing across the board. Similarly for Title I reading/mathematics academic support for at-risk learners. Or, for licenses, it becomes more cost-effective to purchase an unlimited site license than per-student or per-classroom.
Less cynically, it's also because basic skills are not the focus of post-secondary education, and the most commonly restricted accommodations generally bridge basic skills gaps. Ironically, it also reflects litigation history, where families have felt that accommodating too early was depriving their children of opportunities for skill remediation, resulting in institutional reluctance to offer those types of accommodations. (There is, of course, legitimacy to both sides of this argument.)
This question is also why our particular public school decided years ago to make certain accommodations that are often specialized in other schools universal in ours. Our state-wide mandated testing is starting to catch up to that a little bit, and also has added a much wider range of accommodations available without proof of disability than in the past.
But it is still quite variable across universities, since the federal regulations leave implementation very much in the hands of schools.
Also, remember that the age of the student is irrelevant--once they are enrolled in a college/university, you as the parent have no rights whatsoever to access their records or communicate with the school regarding their situation, unless your child expressly gives you written permission. It doesn't matter if the student isn't old enough to drive, vote, or sign a legal document (ironically, since how can they legally consent to you viewing their records?)--or have their own iCloud account, for that matter. So make sure you get all that out of the way for written communications, or be present for all conversations the school has with your child.
