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 think it doesn't really matters if other languages exist. Unlike natural languages, for which you could make a compelling argument that other languages should no longer be taught, multiple programming languages are relatively harmless, given how new the field is. I'd see little value of going ahead and standardizing on Java or C++ right now, as the free market does a much more efficient job than any regulation at making the most competitive technologies emerge.