Yes there is competition for streaming but not for the specific content of the services. With music I can choose Spotify or Deezer or whatever, but for Disney content I can only use Disney+ or for Paramount Content only Paramount+, so they have a monopoly for their respective content if I want to stream it.
That’s most industries though. That’s like saying Toyota has a monopoly on Toyotas but, I can just buy a Ford that does the same exact thing. That’s not what monopoly means.
That’s not a very good analogy, but even if we run with a cars one… then it’s more like one dealership being the only one allowed to sell Toyotas. You aren’t allowed to buy any Toyota, even a used rusty one, from any other dealership. If the dealership decides it costs too much in storage to keep a certain car, like a Prius c, in stock…well they just get rid of the Prius C altogether. (HBO and Westworld being the dealership and Prius c for example)