"Overwhelmingly" may be correct everywhere, or it may be limited to just developed nations — I visited Nairobi a decade ago, and that city varies wildly from "this is very nice" to "this slum appears to have been built on a landfill and the ground is accidentally paved with plastic that was repeatedly trodden into the dirt".
However, even in developed nations, the quantity is large enough that the remainder is an observable issue: around the same time as my visit to Nairobi, 10 years ago, the UK introduced a minimum price for plastic bags (then 5p, increased in 2021 to 10p), to reduce bag usage, because it's just so easy to just not care enough about free things to make sure they end up in landfill (or recycling): https://www.gov.uk/government/publications/single-use-plasti...
Most plastic breaks down into microscopic pieces, which get everywhere including in the human brain in alarming amounts. They get into the human body through food and water.
You haven't seen any reports about this? "Microplastics" does not ring any bells?
>[plastic bags] don't really react chemically with anything in nature
Almost no one denies that "forever chemicals" are toxic to humans even in tiny concentrations even though they are very much chemically inert. By "forever chemicals" I refer to per- and polyfluoroalkyl substances (PFAS) (used in the production of Teflon, Gore-Tex, etc) or more precisely the chemically-stable compounds into which they break down. Just like forever chemicals, microplastics bioaccumulate.
Nothing made of atoms is truly chemically inert, not even noble gases. It's just more or less reactive, and when/how.
But even if it was literally un-reactive, sometimes it's enough to just be in the way. Imagine folding a protein, or assembling a structure of RNA origami*, but some big lump of un-reactive molecule is in the middle — the ultimate shape is different, leading to different biochemical results. Grit in the gears.
Or even just heavy: deuterium is chemically identical to hydrogen, but still has a lethal concentration** because it is twice the mass.
** Replacing 50% of the hydrogen in a multicellular organism with deuterium is generally lethal, unless this is a widely believed myth that's about to get a bunch of debunking
Not all harmful effects are caused by direct chemical reactions. For instance, asbestos causes health problems through the physical process of friction and piercing. Small particles that aren't removed by the body can do a lot of harm.
Two scenarios here:
1) They don't react with anything, meaning the billions of tons we produce keep increasing. Forever.
2) They do react, break down, get into the soil, water, blood, people, and have studied detrimental effects, and many more yet unstudied.
