You've raised very nice question here, Let me put it in simple word. What if I ask you, Why do you use Google though there are many search engines? Probably, You can answer it easily and that'd be the answer of this question as well.
By the way, I am Google crazy guy and I have many solid reasons so I always use Google for everything I need. There are many things about Google which I can't find in other search engines like:
1. The simple Homepage so it takes very less time to get load in comparison to rest, and its simplicity that many other search engines lack.
2. It's easy to use, faster, smarter, lighter, reliable, user friendly.
3. It's a crawler-based engine[software programs designed to crawl the information and to add it to its size-able database]
4. It always takes the best guesses out of your misspell words, it doesn't care about punctuation and grammar.
5. It provides the wackiest auto completes and throws the best in search engine within seconds for you.
6. It has very many awesome tools and features like gamil, image search, Google map, news, ad sense, ad words, locality, auto sense, lots of domain aliases, local business, Google+, Android etc...
7. It indexes the relevant results for us within mili seconds or less.
8. It gives us what really we want even in terms of AD, It is not biased that is the best thing, It always gives priority to great website and the relevant information and that's why our website(TEC) also getting a great amount of traffic from Google only.
"People says 143. I say 444 i.e. Love Your Life"