Production companies can't produce content that quickly. Back in the day we had weekly sitcoms, and it took about a week to produce each episode. But for the most part they all took place in one or just a few locations, and usually those were sets on a sound stage. So all they had to do was churn out a script a week (which is no easy task) using the same characters and same actors. The writers usually had a "show bible" that has elaborate backstories for every character that they could draw from, as well as knowing the actor playing the character and sometimes even getting input from them.

There is a reason people would complain that "every week is the same story!". That's because the constraints of the medium meant that you had to keep deviations to a minimum.

Black Mirror is akin to making a movie for every episode. Every episode has different actors, which means they need to be cast. They have different locations, which means they need to be scouted or built. They have different characters which means a lot more work for writing because every character needs a backstory. Basically they have to start fresh for every episode.

Something like Stranger Things is sort of 1/2 way between -- they have their characters and actors, but they have a lot of locations, and the story varies wildly from episode to episode. Each season is like making a really long movie.

And that's just the production side of it.

From the business side it doesn't make a lot of sense either. The whole point is to provide fresh content for users. If you can provide a new show every month, that's a lot more compelling than a new episode of the same show.