I'm guessing that the plastic is more coming from stuff we eat or inadvertently or purposely put in out mouths, rather than leaching from containers. Just a total guess - but my reasoning is that it seems like you'd get a much higher dose of plastic ingestion by things either in the food supply (including processing and packaging equipment) contaminating the product.
People put things in their mouths all the time too. Toys, instruments, whistles, kazoos - whatever. Also clothing. So many fabrics are made from oil byproducts - such as polyester, fake leather, and more. Your sheets, pajamas, shirts, socks, underwear. We're rolling in that every day.
Toothbrushes, lipstick, ChapStick, and stuff like that too. Seems like you'd get way more plastic into your body these ways.