why does compiler thrown an error when we declare an extra template argument and not used in definition?

I have a below code.

#include <iostream>

template <class T,class U>
T myMax(T x, T y)
{
   return (x > y)? x: y;
}

int main()
{
  std::cout << myMax(3, 7) << std::endl;  // Call myMax for int
  std::cout << myMax(3.0, 7.0) << std::endl; // call myMax for double
  std::cout << myMax('g', 'e') << std::endl;   // call myMax for char
  return 0;
}

On compiling the code, compiler reports an error as show below.

functionTemplates.cpp: In function ‘int main()’:
functionTemplates.cpp:18: error: no matching function for call to ‘myMax(int, int)’
functionTemplates.cpp:19: error: no matching function for call to ‘myMax(double, double)’
functionTemplates.cpp:20: error: no matching function for call to ‘myMax(char, char)’

I know that if i remove class U, compilation will be successful. But i want to know why does compiler bothers about an unused parameter?