Why don't carts have a little handheld scanner, then you pay at a scale?

Pretty much the title. Like, get those handheld scanners and attach them to the carts. I scan items as I put them in, roll up to a "register" where the cart is weighed and verified by a cashier. I just hand over the cash then leave. Or even better, install load sensors in the cart.

Usually I like to pack my groceries into my boxes as I get them into the cart. Keeps things orderly and neat and I also don't buy more than I can carry home. But this means I have to unpack them to place on the belt then pack them all over again after paying. It would be kinda nice to just pay by the cart load.