It's certainly not like "it must always be remade?".
But indeed, a lot of movies from other countries do get remade in Hollywood.
There could be many reasons for this :
1. Better marketing with international stars
Notable example : The Departed, which was a remake of the hong kong
thriller Internal Affairs.
Let's just take this one example, do you think Hollywood would have been able to market the same movie so successfully without the big names like Martin Scorcese, Leonardo Di Caprio, Matt Damon and Jack Nicholson ?
I am not saying that all they do is just add the weight of the names. They are certainly great actors/directors, masters of their craft, and when they reenact a good story, they do make it very nice.
2. Technical Improvements
Hollywood definitely has the best infrastructure in terms of technology used in the movie. From sound mixing and lighting to special effects, Hollywood can certainly make use of the best technology in every scene, which might be lacking in the original movies.
Notable example : the TV scene from the movie Ring(2002), compared to
the original Ringu(1998). The Ring(2002) delivers the scene much
3. English language
English language is the global lingua franca. So releasing a movie in English, definitely increases its prospects of being seen by more people. Of course it can be achieved by dubbing or subtitles, but they are not as effective.
So these are some good reasons, why Hollywood wants to remake these great movies.