The hatch, by necessity, has to be constructed in such as way that the pressure of the depths helps to push it closed, so that it uses the pressure to maintain a better seal.
This is, FWIW, the exact opposite of how aircraft doors work, where the pressure *inside* the cabin pushes the doors against their seals. It's also the reason why you cannot physically open an aircraft door when it's at altitude and the cabin is pressurised.
To design such a hatch so that it opens from the inside requires "through-hulls" (technically "through-hatch") through which the handle mechanisms are run. This introduces an additional point of weakness / leakage. Combined with the fact that there's no reason to open the hatch from the inside, it's an easy call from a design perspective to have a hatch that only opens from the outside.