ECC passively benefits everyone, even people who don't know what it is or why it's useful. Anyone can be a victim of random bit flips, it's not a targeted threat.

Memory encryption, on the other hand, provides absolutely no benefit to 99.999% of users. If you consider yourself to be such a high value target that you suspect someone might gain physical access to your hardware without your knowledge and carry out extremely sophisticated hardware attacks to extract your data, you are a tiny minority and it makes sense that such niche protections would require buying specialized hardware. Even then, the odds of such an attack being chosen instead of a far less sophisticated software-based approach are also tiny.

Of course, if the hardware itself supports the feature and AMD simply decided to disable it, that's still a shitty thing to do, but let's not pretend that it is in any way comparable to ECC.