I believe it was said that each tree would need a "collision box" that would need to be constantly updated relative to all planes in the map and would eventually require so much resources it would bring the sim to a halt.
Rise of flight has collision trees, but look at how many houses we have in CloD compared to ROF, and all of CloDs buildings have collision boxes. So if the same performance rules applies to ROF, they can have collision boxes for their trees because the total number of collision boxes are the same as CloD with ONLY collision boxes for the buildings.
Solution!?
Dunno, but I'm sure that if they find a way to implement it without killing our performance it will be done
I'm just happy we lost the "invisible trees" from IL-2 1946. God I hate those.