And it allows users to create their own one-off objects that they need rather than a corporation creating an immense surplus of parts the majority of which will never reach consumer hands and will end up in a landfill.

This is key. You can 3D print things to fit your exact needs. Mass produced injection molded plastic is only cheap because of the mass production. Molds are expensive. That means they necessarily have to produce a lot more than people need and market them to people who don’t actually need the item in order to make up for the upfront cost.