Sticking to their core competencies would be my guess. I reckon if they thought they could do it better, and make it profitable without endangering or compromising their brand, they would have taken the leap.
Probably though, it boiled down to all the good domain names being taken.
Apple has gained success helped by people's idiocracy of paying 3 times more for a product even if it doesn't bring you something really useful. They are the best in doing that, check the iPad 2 launch features: USB, Webcam or whatever they're putting into it. It's the same as other gadgets have, but they can amazingly sell it 3 times more expensive.
My guess is they've considered joining this business, but beating the already-existing "Big Fishes" would be hard, no matter what.
I would argue that Apple is more hardware-oriented than software-oriented. Although a top-notch search engine is built on a huge mega-bank of hardware in the background, that type of hardware is really outside Apple's consumer- (or even individual-) focused hardware.