That's what makes them so great, and why their games continue to revolutionize the industry standards. Just when you think you've seen their best stuff, they come out and show you something you didn't even think of.
And just when people thought the 80's were dead and that Vice City was going to be a one-and-done game, they came out and released Grant Theft Auto: Vice City Stories.They put this game out on the PSP first as well, in the same vein as Liberty City Stories, which they later ported to the PS2 and eventually the PS3 via the PlayStation Network. Again, they went the prequel route for this game, further proving that their storylines they bring to their games are never just individual experiences, but rather fully evolved, before and after whichever games they are presented in.
In Rockstar's eyes, their universe spans infinitely, and their games only capture specific moments in time of the universe they have created. Their universe doesn't start and stop with each game. Their world continues as it would even without games to showcase it. That's how they present their philosophy in each experience they create, at least.
For being a prequel and a smaller game than the full console experiences, they still decided to evolve the game and gameplay mechanics. There is "empire-building" in the game now, where you can open and operate businesses out of confiscated properties. How deep you go into this mini-game within the game is entirely up to you, but it's nice to see them continuously adding new elements to the tried and true format. In addition to that, when you get arrested or killed, you can now bribe police officers or hospital staff to lower your wanted level and keep guns that normally would have been lost.
Despite initially being reported to be a direct port when they brought it over to the PS2 the following year, Rockstar actually improved graphics and other little performance details, as well as adding a few more side missions and rampages. Again, just when people thought Rockstar would be content with the basic concept of a port, they evolved the idea of what a port can and should be.
Just when the gaming industry thought they had all the answers, Rockstar changed the questions being asked.