Yo: I think that comes from a combination of factors. Personally, I’m optimistic and I feel that I can improve any project that I’m on. Experience has softened this, but I still feel that way. It takes a lot of crap for me to realize that my involvement will not improve the project.
Second is economics. We have attained a certain point in our careers where we know that we are only a few phone calls from another job. However, there is always going to be young designers and struggling consultants with a pile of bills that do not have that luxury.
Lastly, I think many of us (including myself) are a bit soft. Bob Lutz kept emphasizing this in his book from a couple years ago. He kept describing designers as “gentle people”. I think there is a little truth in stereotypes. I think design attracts more introverts than marketing or entrepreneurs (ie our clients) does. So designers get drowned out in meetings and pushed around.