If everyone would buy it early, the next one would be released even earlyer (buggier). It's like you do a sloppy job at work but your boss still gives you a bonus. That doesn't encourage you to work harder next time.
If everyone would buy early, they would have the money to get more programmers to work, which would primarily result in more games/expansions, probably, as quality management surely has exponential costs attached to it, in this genre - meaning after a certain point, it is just better to just release the game, wait for the bugreports to roll in and then start fixing them. There is hardly any way around this, no matter how much the game costs almost. Unless a PI game can be guranteed to roll in three-digit-million dollars in sales, a really thorough QA is simply not economically feasable, prior to release. And that makes PI-gamers more than just costumers in the conventional sense: They dont buy the complete product, as well as they dont expect PI to stop working on it, once the money´s on the table. And if you look at the whole of a game like EU3, you cant really say PI did a sloppy job. It´s just that ´release´ is not equal to ´finished´, with PI. When a PI-game/expansion gets released it might be 3 quarters through production - it needs the community to even start with the fourth in an economically sound manner.