Video games become worthless very quickly. Theres not many games that are still being sold that are more than 10 years old. As Quake 3 has gone through the budget phase, if it were still to be sold it would be one of those 3 for £5.00 games thats barely going to cover the cost of the packaging. By making the game open source, it helps keep it alive. People can make better mods for it and stuff. The source itself is probably a bit too old for people to rip and use for a commercial quality games, but useful to people who want to learn how to code games.
but what has always thrown me is why did they make some of source code ( especially "Quake - Arena" ) open source?