To add on to this: a lot of customers are probably people who have the patience and skill to propogate. (As others have said, propagation isn't really that hard, so it's more about patience and time.) At the same time, if you have the patience and skill to propogate a plant and are still at a greenhouse buying plants, it's probably because you're specifically looking to buy a plant where someone has already done that work for you. You're not going to proplift a plant instead of buying it, because you're there specifically to buy something instead of doing that work.

Propagating plants and growing them from seeds is a longer process that's prone to failure and can take time. It's very common for gardeners to propogate/grow from seed some plants while also buying others that are already more mature in order to eliminate that process or because all their tomatoes this year died and they need new ones.

Sure, maybe every now and again you get someone who's there to buy something else, sees a dropping from a plant they want that they might have otherwise bought, and grabs it, but as you said, that's really not common enough to cut into their margins. You're consistently paying for the saved time and effort when buying plants at greenhouses.

Now, if the greenhouse itself makes a habit of taking fallen plant bits to propogate new ones, that does change things a little bit, because you're taking something they were going to use. But if it's just going into the mulcher, who the fuck cares.