I think you are missing a few important points here.
1. The dismally low level of high school level education. Undergraduate school provides a level of education at the high school level. College graduates tend also to be more mature that HS graduates.
2. Turnover. Unless you apply for low end retail jobs, where high turnover is actually a desirable thing for the employer (keeps the wages down) - most employers do not want high turnover. One reason is that it increases their unemployment insurance tax. Another is that tra8ining new people is a cost to the employer and productivity loss. College graduates are more likely to be "career minded" and stay with the company longer than HS graduates - many of whom want to take a year off and then continue their education.
3. Legal protection. Artificially high entry requirements shield employers from prospective discrimination lawsuits. If most job applicants do not fully meet all the criteria, or meet them barely, the employers have an easy excuse if someone accuses them of bias or discrimination.
Wojtek