The problem is exclusive rights.

If you wanna watch 3 different shows but they are all on different platforms, then you gotta go and pay for all 3. You can’t just watch the Netflix version of Loki, or the Disney+ version of Ted Lasso.

You mentioned monopolies but the problem is that each platform holds hundreds of monopolies, each for one specific show/movie.

In a perfect world, there would be some sort of law or agreement against exclusive rights, where every service can show any product they bought the (non-exclusive) rights to.

In that scenario, streaming services would have to compete by being the cheapest or offering the best service.

But alas, this is not a perfect world