Yeah, but if a game doesn't support it then your total graphics computing power is cut in half. And even in games that do support it, it may fall noticeably short of providing twice the power of a single card. By buying a single card, you can get consistent performance in every game.
A very rough example with made hypothetical numbers: with two cards, you get 50-60 fps in some games (with multi GPU support), and 30 fps in many games that don't support multi GPU. Or you can get 45 fps in every game with a single GPU that cost the same as the other two GPUs. Many people would rather have the latter option, especially as the proportion of games supporting multi GPU shrinks. Again, a very crude example just to convey where I'm coming from.