But things like pesticides are being tested and approved first. With the knowledge they were able to get at the time, they were approved. It was only later, when long term and large scale usage of certain pesticides were shown to have a negative impact on the ecosystem that they looked at them again.

Of course, these things don't live in a vacuum; the manufacturers of e.g. pesticides have a vested financial interest in selling their product, because money. They pay for scientific studies in favor of their product, they schmooze (= bribe) with decision makers and politicians, they overwhelm the system, they take their product global and sell it to whoever is buying, etc.

Same with cigarettes or asbestos or lead paint; it's part "we didn't know" because it's long-term effects or the science wasn't there yet, but part "shut up and buy my product" too.

Anyway, proof that it's harmless is not easy to get in certain cases, not when the effects only show up long-term or when the science doesn't yet know how to test. Was science at a point where they could test for the presence and endocrine effects of microplastics in the human body?