I imagine it's similar to the grocery bag situation. Reusable cotton canvas bags keep plastic out of the environment, but their production emits thousands of times more CO2 per bag than disposable plastic bags. If you buy groceries once a week, you basically have to use the cotton canvas bags the rest of your life to even have a chance at breaking even, everything's a trade-off.

Aren't there better materials to make the resuable bags out of than cotton ?

Also, you can reuse the plastic bags.

My favorites are made of nylon. It's a much lighter material than what most stores are selling as reusable bags. Enough so that you can easily keep 1 or 2 with you all the time if you're the kind of person to carry a purse or courier bag. Meaning that, unlike those canvas bags or the ones made out of that big heavy stiff plastic material, they're convenient enough to actually get used regularly.

I found one study that suggested, though, that the lowest environmental load came from the regular disposable bags as long as you reuse them until they wear out. The main reason I don't do that is that I've discovered that supermarket employees seem to really hate that, in a way they don't with the bags that are made for reuse.

I'm also inclined to guess that it's still bikeshedding. The shopping bag is really visible, but it's maybe a small amount of waste compared to all the unnecessary packaging for the products it's being used to carry home.

How does this hatred manifest?

Maybe because the normal unused plastic bags are in that sheet you can just peel one off and it is easily opened up thanks to the design of the plastic bag holder. While if I was recycling them they are probably a wadded bundle inside another plastic bag.