Remember when bottle caps were NOT an environmental scourge? Kids collected them, played with them, traded them. You didn't find bottle caps in the stomachs of baby albatross chicks, or polluting the beaches of the entire world. In those days, children were not born already poisoned by Bisphenol A.
Bottle caps used to be made of fluted metal with a natural cork liner that formed a seal on a glass bottle. Cork was phased out as a sealant in the 1960s, in favor of—you guessed it—plastic. And glass bottles have all but been phased out, in favor of plastic ones with caps that often have promotional ads printed on the inside. The more bottles you buy, the better your chances to win. Who wants to collect that.
Foamed polyethylene, and something called "Plastisol," a PVC resin plasticizer, are the standard lining materials used in metal closures for vacuum packing glass bottles today. Mostly beer bottles; glass soda bottles are usually produced only for gourmet and nostalgia products.
Do you remember plastic-less bottle caps? It wasn't really that long ago.