"Why does C++ even exist? Why does MS keeps maintaining such a language that caused so many buffer overflows, and generally was not as secure as C# or Vb.NET."

Typical of so many newer programmers, they blame the product for their own shortcomings or ignorance.  c++ allows one write as good or bad code as you want. 

"I was wondering why C++ still exists when C# is that good of a language. MS, why not retire C++, and just focus on C# and Vb.NET and F#?"

Because there is to much production code out there worth billions of dollars.   Most notably WINDOWS!!!

"Its confusing many people, and things needs to be simpler, with few languages. C# for experts, VB.NET for beginners and intermediates. "

C# an expert language?  As someone mentioned try writing a device driver in c#. 

When Microsoft bets the farm on .NET i.e Windows, Office, Exchange Server then it is a viable commercial environment.