The real question should be is why are US textbooks so hard when most people never need HS math after finishing school, and lack so many other skills that are never taught.
It alway baffles me when I see "why don't they teach how to do taxes?". Well they do teach you how to read and hiw to do maths? How hard it is the to put 2 and 2 together?
Taxes and maths are two very different things. The math part of taxes usually is easy, what's complex are the countless rules around it, e.g. what can be counted as a deductible expense and what not - additionally, most rules don't even make logical sense as they were written for political reasons (i.e. to appease one or another voter/corporate interest group).
Thinking is an important skill, which is why scientific experiments, essay writing and philosophy are so important. Integrating equations, not important.
