EDITOR: I bagged groceries for a large supermarket. Plastic carry-out bags were easily the most-efficient vehicle for meeting customers' needs. Placed on a rack, plastic bags are much easier to pack than "reusables" (or even paper bags).
Usually reusables don't stand on their own (if at all), obliging the bagger to use one hand to hold them open while packing with the other. Using reusables takes substantially longer to complete the bagging process; it's nowhere near as efficient and orderly as using plastic.
Reusables get dirty and aren't washable. Health-sanitation issues have multiplied with their increased use.
If the well-heeled environuts who push the schlep-your-own concept were serious, they'd spend some of the vast sums they and their lawyers get from lawsuits they're constantly pursuing to sponsor development of processes for converting plastic bags back to the petroleum from which they were made.
But they're less concerned with the environment than they are with their hatred not only for oil companies but also for petroleum itself — in the present era, civilization's lifeblood.
The packed-on-a-rack, plastic carryout bag revolutionized the grocery industry. Prepare now for longer, slower checkout lines.