> We test for things that are really dangerous (e. coli, arsenic, heavy metals, etc.). If microplastics were dangerous like those things, it would be pretty obvious by now.

Microplastics aren't necessarily dangerous like those things, but they've been shown to be potentially dangerous in different ways. Really acute problems of the sort you're thinking of, like long-term cancer risk and short-term infection risk, can be considered alongside reduced fertility in men and developmental problems in infants and children.