a) People would bitch more because it would take forever to get into an airport. And,
b) It isn't practical. What's next, stopping you at your car or bus before you park?
People demand security but don't want inconvenience. And if security fails, they point and say "it's ineffective." Even if it stops 80% of attacks, unless it's 100% people will bitch. AND if there aren't any attacks, can they really claim that they stopped one unless they publicly acknowledge every attempted attack?