But the chips are identical AND PCBs are compatible! I've seen many non-ECC modules with just one chip not soldered on. Unbuffered, of course.

It's 3-5 orders difference in the volume (and most of the time people buy high capacity modules) and ECC errors makes very obvious the need of replacing the module, while unregistered errors on non-ECC modules are just other 'something gone wrong' type and couldn't be diagnosed easily.

Also, onr of the primary markets for ECC UDIMMs are server/pro-workstation, thia alone.adds at least 10%