“Because that is the Minhag.”
I once asked a friend of mine, who keeps gebrux, about saliva in the mouth. After all, saliva is machmits. He said to me ain hachi nami, but that’s the minhag. Some do apply some svara to the issue of gebrux. For example, my wife’s family would wet matsa with pure OJ, not from concentrate. I see it as a sfek afaika for those that keep gebrux.
If you want to stop keeping it, ask a Rav.