Yes this becomes more, and more of an issue the higher in frequency you get.
"But I play bass, so this doesnt matter..."
If you play with distortion, which adds more content your up upper harmonics, then it matters. And it does effect guitards, and worse than us bassists. Ever go to a show and stand right in front of a 412? It will cut your head off, move over to the side a bit, and it gets a lot better. It is also part of why guitards over power the mix, yet insists that they are not too loud.
As for why they do it? Guitar is steeped in tradition, voodoo, and superstition. They dont know better, and dont want to hear anything besides, "Get a marshall, and two 412's"