> I haven't seen any of these tools doing anything that anki doesn't — why not contribute to it instead?

For the same reason that your area likely has more than 1 pizza restaurant. Other people want to take a stab at their own vision of a thing and that's awesome. Personally, I find Anki to be pretty uninviting to the general public.

Fluent-Forever.com is an example of an application of spaced time learning applied specifically to languages and their app is pretty great.

We may disagree philosophically, and that's ok, but I tend not to buy into the perspective that if X already exists and Y is like X then Y should not exist.