I think people are so feral in the way they talk about microplastics (in ways that they are notably not feral about much more demonstrably hazardous pollutants like heavy metals) specifically because Western culture is steeped in this idea of the body as an inviolate perfect thing separate from its environment over which we exert control.
This is, of course, nonsense; the body is just a mass of random garbage from its environment that has contingently and temporarily clumped together. You're made out of stuff from the environment, of course everything that's on the environment is in you.
But it's an ingrained, culturally-bound illusion, and people seem to encounter as body horror the idea that they aren't, like, a pristine divine entity that exists within a membrane, apart from the grimy world. In particular there's horror at the idea of an inescapable invader to the body, something that's in you (even if, really, whatever effects it has are probably marginal to a million other things going on in your life) and therefore that you can't put into the box of things that happen to other people – the same box that the average young person online uses to hold concepts like 'illness' or 'fatness' or 'disability', you know.