I don't see why the ability for a hobbyist with hobbyist equipment to assemble a very particular lens with a ton of caveats should have any impact at all on the prices of a full consumer product with a vast amount of benefits and features.
Which is not to say that lenses are not overpriced, but the fact that you can theoretically 3d print your own... anything (gun? Car parts? Doorknobs?) doesn't imply anything can or should happen to the price of a much more functionally complete product.
Indeed, most of those things you listed don't make sense to 3D print because they're mass produced which is cheaper with injection molding. I'm sure you can also buy a cheap low quality mass produced camera lens too. 3D printing's advantage is in unique things.
Just thinking of getting the focal plane of a DIY lens pan on the sensor plane requires more precision manufacturing then is available for the DIY crowd. A relescooe, sure. And even then you would look at sub-pat optical results. A camera lense? Fun project for sure, but nowhere near eveb the cheapest lebses on the market. Let alone good condition vintage lenses from the prime manufacturers and their third party alternatives.
Which is not to say that lenses are not overpriced, but the fact that you can theoretically 3d print your own... anything (gun? Car parts? Doorknobs?) doesn't imply anything can or should happen to the price of a much more functionally complete product.