Here's the thing about that:
lots of particles have been found in the placenta[1]. In the case of the study that found microplastics[2], they found 12 particles in 4% of the volume. 12 - total. So maybe 300 total particles. In about 600 total grams of placenta. It's difficult to put a scale on how few that actually is.
So the issue here is we're not really talking about a unique threat, just a vector of an existing one. In terms of bioactive compounds which could find their way to the placenta, metals and carbonaceous soot are going to be a lot worse then inert plastics (of note: the paper involved notes that the primary possible mechanism is the microplastic acting as a carrier delivering bioactive molecule - not the material itself).
It is also worth noting that in the same study, 2 of the studied placental samples showed no microplastic contamination. Which is to say, given the transport mechanism proposed - inhalation of particles - it's quite likely that the medium is not things ingested in the GI tract, but rather airborne contaminants.
Which would make a lot more sense, seeing as how if micro or nanoparticles were routingly migrating through the gut, we'd suffer serious problems with bacterial infection, or just dealing with something like soluble fiber (which is essentially a plastic - long chain cellulose - that isn't digested).
So the question isn't "why not avoid it?" it's, "are you even avoiding it?" The GI tract is a relatively robust system that's very good at rejecting the external world from it. Whereas your lungs are pretty much a direct link to the blood from the atmosphere.
[1] https://www.qmul.ac.uk/media/news/2020/smd/air-pollution-par...
[2] https://www.sciencedirect.com/science/article/pii/S016041202...