we can't? I thought that stellar wind drops off so steeply due to 1/r2
that compared to the distances between stars, it would be irrelevant. Or at least I've never heard any of my professors bring this up as an important concept, nor have I read of stellar wind pressure being important on galactic scales.
I may just be wrong though, I'm not a part of the astrophysics community so it is very likely that I could be unaware of that kind of idea.