Why std::get uses template parameter instead of regular parameter?

Why std::get uses template parameter instead of regular parameter?

The std::get function uses template parameter as a regular parameter.
As an example:
std::tuple <int, double, long &, const char *> bar(18, 6.5, 12, "Hello!");
cout << std::get<2>(bar); // Print the tuple's third element.
Why this function designed this way?
Why not using a regular parameter instead?
For example, something like: std::get(bar, 2) or std::get(2, bar)