Because DRM is primarily used to ensure the content is not shared in a way the owner does not allow. That is not what SynthID is doing. All it does is allow people to know it is a generated image specifically for when it starts to be widely shared on the internet.

So strictly speaking brings a lot to the discussion when you actually think about it. Stating that DRM != SynthID is addressing issues where people seem to think that DRM == SynthID. Those people are wrong, and strictly speaking need to be corrected.