why so many languages

wouldn't it be better if there was only one language like c++ or c#
then programmers could spend their time mastering that language
instead of re-learning the basics of another language every few years

i find that in general, new languages are easy to learn as long as you know similar languages.
for instance, if you know C++ or java, you should be able to sit down and write C# code with just a little bit of guesswork and MSDN