Well, building a search engine today is no big deal. Even a high school student carrying good grasp in computer sciences and programming can create his own search engine and let it running. However, the main thing that stops apple from creating its own search engine is the unique Search Engine Ranking algorithm which Google uses.
Currently, this is the best search engine ranking algorithm that is out there which makes google the top contender among all search engines such as Yahoo, Bing, and MSN. Moreover, google puts constant effort in improving the search engine so that it stays above in the game!