Because the question is actually misleading.
Beta testing of any game that isn't on a server is very tedious.
Beta testing is done to iron out problems that occur with different computer set ups, finding things that have been overlooked, and to stress-test the program. The last part isn't that important with non-server games, which means you are looking mainly for CTDs. This also mean that, if done properly, you will be playing the same thing a gazillion times, just to make sure that the CTD is reproducible and/or that it wont happen.
You will also have to play a certain way, i.e. try all the civs with advanced start, or whatever.
Most people don't know how beta testing is done. And while an open beta of, say, WoW can be fun, beta testing Civ is an entirely different beast. With the WoW open beta, you were mostly stress testing, which means that you actually got to play the game.
You do beta because you want to help with the game and see what it offers ASAP, not to play it nor have fun.
This, and what Chalks said, except I don't want into Civ beta, as I simply don't have the time required anymore (or they would have to pay me a fair bit more than what I make now).