Because you cannot use argument deduction for constructors. You need to write explicitly std::tuple<int, double>(i,d);.
It makes it more convenient for creating a tuple and passing it to another function in one-shot.
takes_tuple(make_tuple(i,d)) vs takes_tuple(tuple<int,double>(i,d)).
One less place to change when the type of i or d changes, especially if there were possible conversions to between the old and new types.
If it were possible to write std::tuple(i,d);, make_* would (probably) be redundant.
(Don’t ask why here. Maybe for similar reasons why syntax A a(); does not invoke a default constructor. There are some painful c++ syntax peculiarities.)
UPDATE NOTE:
As Daniel rightly notices, c++17 will be enhanced, so that template argument deduction will work for constructors, and such delegation will become obsolete.