In the GM games I've watch it's very rare to see a game ending in less than 10 mins. They seem to like going two base first before attacking.
Even if one player is Zerg, they still go with the early two bases and the Zerg doesn't ling rush at all, even if it seems like a logical choice considering the opponent won't have many units; lings can easily destroy the second base.
Are there special rules in GM games (no rush) or do they just like to have a real war game (mass units).
As far as I'm aware, there aren't any official rules about rushing. Early game rushes and all-ins don't work nearly as well against the high level players as they do against the weaker or casual players. A player who has very good control over his drones can easily defeat a 6 pool, for example.
Also, rushing is very hard on the economy, for no guaranteed reward. You say that the lings could easily destroy the second base, but that's not always true. Unless the opponent is doing a high risk early second base, such as Nexus first protoss, then they almost always will have at least some way to try and defend such an attack. If the rusher doesn't do significant damage, then they are stuck with fewer workers, wasted resources on the assault, and only on a single base, which means a significantly harder middle game.
The size of the map is also a large factor, as well as how early the player scouts and gets scouted. With larger maps, rushes become very risky as it takes longer to reach the opponent's base, giving them time to get units of their own, not to mention the time it takes to scout out said opponent's position.
There are certainly more factors involved, including the players' predisposition to rushes, but the players that reach that level of play have generally done so by mastering a combination of macro and micro. Rushes get punished much more frequently, and so most players don't believe that the risk is worth the reward at that point.
They generally scout and adapt to what the opponent is building and an early push is easy to figure out from the scouting so they can build up a defense for it. As for why they don't send a couple zerglings in to take out some economy..Its unlikely to win the game and those 6 zerglings will cost them drones and whatever minerals those drones gather. If the zerglings fail they will be out the minerals they spent on them, as well as the minerals they lost by going for a quick spawning pool. Also, the time it takes to rush (run the units across the map) would allow the defender time to build up units to defend.
In the end, they're generally thinking more on their economy, and having a force to defend an early push is in line with building units for their push and helping their economy.
The main reason is that rushing is a very high-risk strategy that sacrifices the players economy for an early win. Against low level players this can work fine, but high level players are much better at scouting, and will be able to see a rush coming before the units are even made, and can prepare for it. Due to this high level players usually won't even bother with it, since more often than not it won't work.
Something else you need to consider is that a longer a game lasts, the larger of an advantage a player with the superior economy will have. Many high-level games normally consist of small skirmishes and harassment in order to either maintain or obtain a superior economy over that of your opponent.
While everything the other people have said is true to a certain extent, it is important to note that High level players do actually rush quite a bit. However, many pro/GM level players who you see streaming won't do it on the ladder because their win rate will be higher simply by practicing solid mechanics. Also, to a pro, a win on the ladder doesn't really mean anything to them, they are more concerned about the practice they get. Most early cheese and allins don't really require much practice to execute, thus, it is a waste of time to spend time on the ladder doing that.
However, in a pro match (especially in Best of 'x' matches), many pros will throw in an early cheese or allin to gain an easy win and advantage, make up for a perceived difference of skill, or simply change up their build. I don't remember the tournament exactly (i think it was MLG), but game number 2 of the finals (i think it was) between DongRaeGu and Alicia (on TalDarim Alter) opened with DRG 6 pooling. A macro game resulted. In this particular instance, DRG was up on wins and felt the risk of the 6 pool was minimal and wanted to change up his opening to hopefully throw Alicia.
In conclusion, you don't see many GM's streaming ladder cheeses and allins mostly because, in general, they ladder to practice, not to win.
The reason why they don't rush is simply because it does not work. You would have to get lucky to do much, this is assuming they don't have any forces. If you lose the rush, you lose the game. It is much smarter to wait and build decently strong armies than rely on the opponent not preparing for a rush. Zerglings aren't strong enough, zealots cost too much, and marines don't do well either.
Rushing never works. Try rushing a cheater, it's insane. It does not end well. Same with elites. I can beat elites with ease -- but I never rush, ever. With pros rushing is a bad idea. The units are never made for rushing. Scarlet, one of the world's best (ranked wise) players, doesn't rush even when she played Zerg. It just doesn't work. The dynamics go way against rushing. And no, there is no anti rush rule. In conclusion rushing never works against a decent person.
