If that was the case, the a full groove bearing or even a 3/4 groove bearing would work. And it does. Up to the point the RPM, the oil viscosity and loads need the oil there at full flow and pressure.
I have no doubt that grooving the bearing can help. But, if the theory of just slotting the bearing or using a recess where the main feed and the cam feed comes together was a fix, you would just have the hole in the bearing at 12 o’clock and that would be the fix.
Then you can use 1/2 groove bearings and not have the hassle of a crossover or anything else. Any time you force the oil to make a course with more kinks, and tighter kinks, the less effective it is.