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...
They're inert long chain polymers, and amongst other things you need to show bioavailability in the area t you propose would cause damage at sufficient concentrations.
In the current case: how efficiently do nanoplastics cross from the digestive system to the blood stream, how long do they persist before being filtered out by the kidneys, do they retain their structure in the blood stream unchanged, is their a lower limit on toxicity.
i.e. injecting someone with enough pure water will also damage their organs, exposing kidney cells to silica dust in solution in huge concentration would also kill them but there's no way for it to get there.