You’re looking too narrowly at this. It’s not because it supports / doesn’t support sideloading. It works like an appliance. You can forget about it and not have to babysit it. You don’t have to worry about it getting messed up.
Think of it this way - why do Sony or MS sell any consoles? Every person who buys a console could buy a PC and get better graphics, more options, different games stores, more control options, etc.. The reason is (some) people don’t want that. They want to sit down on a couch, select a game and KNOW it will play, no fuss. They don’t care about all that “freedom” they get from gaming on a PC. The walled garden is a feature of some products.