A similar question has been asked on Stack Overflow.
In the accepted answer, Nicol Bolas points out that it isn't in the standard because it has to be proposed and voted on. In the way that C++ works, people write proposals for changes, which are voted on. However, he points out that it's a convenience function and doesn't add that much value over the current methods.
In another answer, iammilind suggests that it's against C++'s philosophy. This would add a dependency to the
<vector> libraries. The current implementation uses built-in and independent types, which doesn't force the library on people. In addition, if a system didn't implement the library, then they would be unable to start the application.
Other answers are also provided, but the two mentioned above are the highest scoring.