The why is that plastic is an extremely convenient, cost effective way to make lots of things. And the evidence that it was deleterious to human health was negligible.

And to be fair, it's still fairly uncertain. We demonstrated endocrine problems with BPA, but aside from that microplastic consequences on health still seems uncertain. At best we're mostly doing the correlation/causation thing that leads people down a confusing path of cure-alls and snake oil.

If there was a smoking gun for the consequences of this in our day to day living, surely it would be regulated out of existence[1], but thus far that evidence doesn't exist.

[1] - ha ha, who am I kidding. In reality industry groups would muddy the waters, try to pretend it's "political", finance astroturfing groups, and soon enough a certain segment of society will be proudly clutching onto their microplastics, demanding higher dose services, and ascribing it with magical cure-all powers.