Plastic bottles weigh less and can be made thinner. Transportation costs dominate the cost of most consumables
That was surprising to me and, uh, no, they don't, unless you're considering building materials (i.e. rocks).
It's not easy to find numbers, but for milk in Germany, it appears to be around 10%, about the same as packaging. Milk should be worse than many other products because it's cheap and heavy, on the other hand it's rarely shipped over great distances. [1]
Another report [2] considers transportation costs relative to the value of the goods transported, which isn't exactly the same as "how much of the consumer price is transportation", but surely related, and has 2.7% for foodstuffs (and 55% for rocks).
I mean, I'm sure it varies widely for different kinds of consumables, but overall, transporting stuff is cheap. I think even in terms of CO2, which is certainly underrepresented in cost currently, production dominates transport for most goods in a grocery store, apart from produce shipped by air.
[1] https://www.agrarheute.com/tier/rind/78-cent-fuer-trinkmilch...
[2] https://bmdv.bund.de/SharedDocs/DE/Anlage/G/MKS-Wissenschaft... (Tabelle 8)
The local milkman of the 50s, did better with glass collecting and delivering milk. The national enterprise benefits, as you say, from weight reduction.
I'm sure glass is far more environmentally sound, but to claim that glass production, glass cleaning(and the harsh chemicals and their production cost to the environment), and water for flushing afterwards are not relevant, would be wrong.
So every business off-loads some environmental impact to the tax payer.
My point? While your statement is true, it has nothing to do with how the business decision is made re: cost to the business. The metric used there is "what saves the business money".
The common 2kg×6 PET bottles format is too heavy for glass bottles, so you end up paying more for heavier 1.5kgx6 bottles that are going to last you less.
IMO it's simply habits
no one would buy beer or wine in a plastic bottle, because it feels wrong (and it is if you ask me).
Here in Italy nobody in their right mind buys tomato sauce in plastic bottles, our granmas would come to haunt us at night if we did.
but we buy water in plastic bottles ¯\_(ツ)_/¯
Where I come from beers come almost exclusively in glass bottles for the large ones (66cl), aluminum cans are only used for the small ones (33cl) but you can buy them in glass as well.
There is usually no price difference whatsoever
For example
https://www.amazon.it/Peroni-Italiano-Moderatamente-Gradazio...
https://www.amazon.it/Peroni-175-Anniversario-Birra-Cassa-dp...
Many of the cheapest beers are imports, for example Bavaria, that you usually find in aluminum cans in Holland but in glass bottles in Italy.
I can't explain why, but for us some things must be bottled in glass or they aren't "good".
Tomatoes is another example: you can find cans of tomato pulp or peeled tomatoes, but not the sauce, it always comes in glass bottles (it exists of course, but it's a very weird choice to us).
Short term benefit and profit margin dominate the calculation of if life environments deserve to continue unpolluted or they deserve to be destroyed in the name of compound interest.