Neither does SMS for large parts of the world. Not everyone has unlimited SMS like US carriers do. And/or as soon as one participant is abroad you're talking international SMS prices.
This isn't a clean fallback option.
However it's a _good enough_ fallback that has been working for many groups. Yes, it doesn't solve the issue for all possible groups. But Apple likely doesn't care yet, and probably shouldn't until there are other changes it can work on that would affect larger groups (like bringing FaceTime to the web).
Most of the world uses WhatsApp though, so there's no fallback necessary if most people in the world never even open up iMessage.