Why can't there be a drop-down selection for sizes and weights, instead of listing everything separately?

It's actually easier on everyone the way it is. Yes, the grower could provide a pick list of sizes for each product, but then they'd also have to maintain a quantity for each of those choices. And they'd have to do that for each of their products, because the sizes for a chuck roast are going to be different from the sizes for a ribeye steak. And you might have a technically savvy producer, or maybe even several, but do you really trust all of them to be able to keep a deeply nested set of options straight on the site?

As it is, they merely have to copy the product (with one click) and then edit the weight and quantity. It's all there in the open, and easy to keep straight. And if they name things well (and you enforce that at your market), it is easy for the customers to scroll through and find exactly the size they want, without having to navigate drop downs to see if why they really want is or isn't actually available.

To give an example from the real world, imagine going to the grocery store to buy meat. Right now, everything is laid out in the case in front of you, and you just pick up what you want. No one says that's too overwhelming. But the store could tidy things up a lot by just setting out one of everything, one chuck roast, one ribeye, one pack of ground, one cube steak, etc., and then you could ask the guy behind the counter if they have one that's 1 1/2 pounds, or boneless, or whatever. Or maybe have a little book you'd flip through that they'd have to keep updated. It'd make the meat section much neater and smaller, but it would be harder for everyone to actually use.

So that's why I designed it the way i have. After using and watching many other people use traditional eCommerce packages that work the way you've described, with drop-downs and so forth, I decided to go for the easier, simpler approach, even if that means the listings are longer than they could be.