"Unprepared" is a euphemism with the hidden assumption that everybody, if "prepared," would be higher ed material. Of course, there are countless reasons why the assumption is wrong, but let's start with some of the obvious ones: half of the population has below-average IQ; many people lack the interest or the character traits needed for rigorous high ed (or for high school for that matter, but may want the diploma); many kids float through secondary school while benefiting minimally.
So Why Do Colleges Admit Students Who Can’t Do the Work?