The reason is simple. It's completely passive. In order to learn, kids have to actively participate.
Seriously though, I've learned a lot just from reading and watching media growing up, in some cases much more than school because it's something I actively wanted to do. For me, doesn't matter whether it's active or passive, if it's something I enjoy I can learn from it much easier.
I totally understand that people want to believe that EC and other "educational" TV shows work, because it is effortless for the parent, the teacher, and the child. But the very nature of it being effortless leads to it not working.
Heck, I'd absolutely love it if watching an exercise video would add muscle mass. Sadly, it doesn't.
And yet people still give lectures, and Youtube tutorials have grown ubiquitous. Because regardless of how efficient uptake is, it's very efficient for the person doing the teaching.
And these old shows were very clearly presented as a jumping off point. I asked to go to the library all the time as a kid because Reading Rainbow and Mr. Rogers talked about how great the library was.
That's why classrooms come with exercises.
Stipulated that in an ideal world, children should watch no television.
But if the alternative is, I don't know, reruns of Leave it to Beaver?