The answer is based only on my own opinions, since I cannot speak for anyone else besides myself when it comes to this. However, my reason for using Google as a search engine boils down to the following things:

1) It gives the best results. I have tried other search engines, and none of them have given me as relevant results as Google is able to. And finding the content I want is my number 1 priority when I use a search engine, so this point holds a lot of weight. 

2) I'm used to it. I have used Google for many years, and I'm very familiar with it. I might try some new options as they come along, but none have really felt right for me. However, as stated above, I will be changing to another search engine if they can actually provide better results than Google. 

3) It works really well in several languages. If I want to search in Norwegian, I can just type in Norwegian, and I don't need to specify it in the settings, since Google will automatically be able to tell. Some other search engines also have this feature, but far from all. 

So, these are my main three reasons. I'm sure other people have other reasons for using Google, but I also think these points are pretty similar to how many other feel as well.