The answer is that a fair number of the teachers they had in K-12 told them that they were bad at math, often in almost boastful terms, and consequently they learned that it is more socially acceptable to assert poverty of mathematical ability than to go through effort to acquire that ability. (Kind of a race-neutral version of not getting good grades to avoid 'acting white'.)
Maybe so.
I was a math teacher for many years and when parent conferences came around and I was not preaching to the choir, many parents readily acknowledged that they were poor at math. And, would say so in the presence of their children. I took that to mean that the parents didn't necessarily expect their children to be proficient either.