Because then it would make switching cl*censored*es basically pointless. There needs to be a reason to have the different cl*censored*es ^-^
Besides, ranchers can have merchant shops with as many items as they want. Just stock your shop to the bursting point as a merchant, switch to rancher, and there you go, merchant shop as a rancher. Granted, you can't take tu out or change prices or put items in or take them out until you switch back to merchant, but you've got yourself a shop that's accumulating tu for you to have. The same thing works for merchants and rancher shops.