Perhaps you're thinking our body is at equilibrium with the amount of plastic in our environment, but the reality may be that our body accumulates microplastics from the environment and they become concentrated over time. Kind of like how we can't get rid of heavy metals from our body, so eating lots of fish accumulates mercury to toxic levels. But eating fish is a conscious decision whereas microplastic exposure is an unavoidable fact of life now.
You can probably reduce microplastic consumption, but it is quite a pain and more expensive. Try buying everything at the store not wrapped/bottled in plastic. This does not even work as well as you might think as many things are wrapped in plastic and then presented as if they were not. For example, breakfast sausages in the meat display at my local Whole Foods are wrapped in paper when they give it to you, but come to the store in ~1lb plastic wrapped packages.