Why so many programming languages for a Computer?

Computers always attract me. In my school time I like to play new games and want to finish it first. Thats start of my thinking about computer science. Then I decided to do my Bachelor in Computer Science.I always like to do Math questions and I think Computer Science is nothing but Algorithms and solve problems. Then I finish my bachelor and decided to do Job. So I looked in the market and found that there are lots of different language and different tools. It was very difficult to decide what to choose. As per Wikipedia there are near about 2000 -2500 languages. So why there are so many languages. All programs are going to run on a machine i.e Computer. It should be better if we have one language. I think thats because of everyone wants monopoly in software industry, they want that only their software should be used.
But it would be more productive, more easy if there would only one programming language and that would be used in all machines, platforms such as Windows, Linux, Mac.