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.

Memory encryption can help mitigate much lower level attacks such as row hammer, these attacks get patched even average consumer devices.

No benefit for 99%? people said the same about FDE. Just as there is not a good enough excuse to not validate integrity and availability of data, it is not for confidentiality when its very much technically possible to do so.