11D Supergravity is a field theory, while the varieties of superstring theories is not. Why would M-theory include theories of two different framework?
The five superstring theories correspond uniquely to the five possible SUGRA theories in 10d as their low-energy effective theories.
The five SUGRA theories can be obtained by various styles of dimensional reduction from the unique 11d SUGRA theory.
Therefore, M-theory is conjectured to be a "stringy" 11d theory that can be reduced to the five superstring theories in similar limits and that has 11d SUGRA as its low-energy effective theory.