China has never built internal weapons bay for fighter before J-20 either.
There is always a first time for everything.
And I don't see a technical reason for a bomber's bay being more challenging than its fighter counterpart. Remember, without speed depth and weight aren't problems, the aerodynamic impact when opened especially the shock from incoming air flow is the challenge. While bomber's deeper bay is a thing, fighter's much higher speed is even more challenging in terms of stability of the aircraft and stress to the structure.