Because seeing flashy colors and collecting points and achievements does not equate to deep meaningful learning. It does help give a little boost to a gaming addiction though.

Also reading/writing are abstract letters on a screen or paper. Listening/speaking is sort of more natural for humans, but I guess it can be thought of as "abstract" because in the end it's just sound waves that we receive and our brain makes it have meaning for us in our head. They are different skills that are all related but not easily transferable. You can improve on one aspect of a Language, like reading/writing but not be able to listen or speak well.

Babies learn to listen and speak, before reading and writing. 

Adults tend to learn all of them at the same time.

In the end all the resources are helpful if you actually want to learn (because you will filter out the good and the bad, and if you are slow learner then how to adapt the learning to you) Vs you want to be the DuoLingo 10 year streak complaining on Reddit that you still can't order at a shop

I chose the path of the 1 million A0 Kwizzes on Kwiziq so I don't care