I'm a compiler engineer on the Q# team - to add to what others have said, Q# is more than a set of quantum APIs, and the Q# compiler does quantum-specific checks and transformations on the AST representation of your program.
To do the same thing in an existing language, you'd need a deeply embedded DSL so that the program can be manipulated by the Q# "compiler" before running it. It's possible, but it can get kind of clunky, where you have to use the "embedded" if statements, for loops, etc. instead of the ones from the host language. A new language helps with having a cleaner separation.