As designers/developers, I think we're incredibly uneasy with saying "I don't know". In a knowledge-based economy/industry, your knowledge is your currency, so I think there's pressure to appear like you know everything. A combination of being used to presenting a good front to the clients, looking up to rock stars in our industry who can "do it all", and general pride perhaps. The industry also changes so fast, new standards, new technologies, and there's always a fear of falling behind the curve.
I think that's a lot of weight to be carrying around, and we don't always realize it's there.