Why do Grand Master players not seem to rush at all?

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.