Gameplay reasons (visibility) might be part of it.I still wonder why in wet conditions the spray behind the cars are so small. These sprays further have only minimal impact on the window of the car behind...as everything else in wet conditions is so well made and life like, this stands really out to me.
The high graphical cost of large transparencies is without a doubt also a factor.
As for having a minimal impact on windows and things... it's pretty common for transparencies to not play nicely with screen-space reflections. (Destiny's SSR method should work okay with transparencies, but it's also using an ultra-simplified and somewhat inaccurate approach to the "ray tracing.")