Why do productivity modules only work on intermediates?

Ok, I remember the process with modules quite clearly. In the very old days, I just added modules as a pet project to add more variability to the game, and instantly started to play with that.

In this version, you had beacons and module slots, and all modules could go anywhere.

It very quickly got to the point, where the best and only setups was 12 beacons with productivity modules around an assambler, to maximize the productivity as much as possible (because of the exponential aspect of it with the length of production chain).

It was clear, that it would affect the game too much this way, so we made the first limitation of productivity modules not going into beacons.

We played again, it was better, but in some cases, it was just weird, for example, we had this recipe where you made rocket, and another recipe, which had rocket + some extra explosives as an input to make the explosive rocket. Basically an upgrade recipe. But suddenly, with the productivity module, it just didn't make sense, I take one rocket, and upgrade it into 2 rockets of a better type. It could also mean, that maybe the better rocket actually gets cheaper at some point, it just didn't make sense and we were trying to figure it out.

This is where the idea with the intermediates came. The intermediates are generally more abstractly related to each other and to the final products, so the productivity doesn't make things look too weird, and it limits the usage of productivity modules enough to not make it "the only way to build for everything".

Later on, we made some changes and shulffed some things around to make sure that everything that is in the "intermediates" tab is the only group of recipes affectable by productivity. (Science packs are in intermediates for this reason if I remember)