Is it that their parents are indifferent to their kids' education? Or they don't realise that calculus is important in many fields? Or they just want their kids to become a lawyer?

I am not aware of any surveys, but I would guess that many parents think it is the school's job to motivate students to study further math, while many schools think their only job is to teach enough students to pass the basic standardized tests, and neither side has properly communicated their beliefs.

Other parents likely believe that further math isn't necessary because they have done well without it and think their children will as well. They do not understand that a large portion of the world that was impoverished when they were growing up now have functioning education systems and are now able to compete for the same cushy jobs.