OpenGL has native support for sRGB blending, so maybe keeping the same gamma allows them to use the same hardware to accelerate Display P3 blending.

Though I think sRGB's gamma is technically the much more complicated

       {  cs / 12.92,                 cs <= 0.04045
  cl = {
       {  ((cs + 0.055)/1.055)^2.4,   cs >  0.04045

and not exactly 2.2.