Because chemical effects are not the only undesirable effects something can have. E.g., mechanical, electrical.
In any case, in one study [0], "researchers looked at 12 brain samples from people who had died with dementia, including Alzheimer’s disease. These brains contained up to 10 times more plastic by weight than healthy samples." Another [1] found "nanoplastics accelerate the aggregation of β-amyloid peptides" and that they exacerbate "the neurotoxicity induced by low-concentration peptides".