Because employers should be able to assume that a high school graduate has a minimal level of competence in basic skills that are required by employers. It used to be that employers could give skills tests to prospective employees.
Then the lawsuits started when it turned out that most minority graduates couldn’t pass the skills tests.
This is how we got so far down the road of “everyone should go to college” crap. The fact that some bozo has a BA has become a substitute for a competence test.
Most jobs for employers only need the basic skills of basic math such as addition, multiplication, division and decimals.
In fact, there are very few entry level jobs for graduating high school students that require algebra. Name one if you can. I guarantee it will be the exception, not the rule.