It's a valid question if you're not familiar with the RAM market. Sorry you're getting downvoted for it.

The manufacturers make the individual chips, not the modules (DIMMs). (EDIT: Some companies that make chips may also have business units that sell DIMMS, to be pedantic.)

The R in RDIMM means register, aka buffer. It's a separate chip that buffers the signals between the memory chips and the controller.

Even ECC modules use regular memory chips, but with extra chips added for the ECC capacity.

It can be confusing. The key thing to remember is that the price is driven by the price of the chips. The companies that make DIMMs are buying chips in bulk and integrating them on to PCBs.