String theory solves the high energy gravity problem by making it mushier:
Lisa Randall - Warped Passages - 14 - String Theory’s Origins
Strings—unlike quarks—have no hard scattering processes. They have more “mushy” interactions that take place over an extended region. This property means that string theory could potentially solve the problem of the graviton’s ridiculously high interaction rate, and correctly predict high-energy graviton interactions.
why isn't this mushy gravity interaction enough to describe high energy gravity without holography? what string gravity calculation can only be done with the help of holography? the answer structure should be: string gravity is flawed because... holography improves it because...
Holography is projecting the mushy string graviton interaction from a $d-1$ dimensions hard gluon interaction, as discussed in this PDF:
The two corresponding lumps of energy modify the virtual cloud of gluons surrounding them, which in turn induces a net attraction between the lumps, precisely reproducing the correct gravitational force. In every physical sense, gravity and the extra direction of space making up the inside of the box do indeed emerge “holographically,” from the dynamics of the theory that lives fundamentally on the walls. This correspondence gives us our first concrete clue as to how space-time may emerge from more primitive building blocks.