Re: Why not a unified catch-up service?
There are a fair share of independent production companies in the US, too. Some of them can be quite big like Fremantle Media, a familiar name with ALL the networks. Many of these can make pilots and go fishing with the networks to get a contract. Such as it goes, but that also means they work under contract until released, so any stuff that goes to a network stays with a network (it's the rule--the publisher takes precedence over the producer regarding ownership). But at the same time, the big networks make sure to maintain their own cadre of studios to produce what might best be called their "core lineup". CBS's Television City, NBC's Rockefeller Center. Even the BBC maintains its own studios, as I doubt they'd trust anyplace else to produce Doctor Who and so on. When it comes to private networks and producers, there can be give and take, and each situation can be different. A network can frequently contract one of their productions out to one of the producers on the condition they do it in their studios.
Going back to your argument of producers going it solo, the money usually isn't there. Television production isn't cheap. That's why the pilot system and the studios and contracts and so on. Even the second tier of television, the syndication system, involves contracts with the syndicate. Only a wholly-homegrown program can be put online with no strings attached.