It's called P r o f i l i n g
Something which we are apparently loath to do less we are considered racist, phobic or anti-whatever. The "let's treat everyone the same" approach is a waste of time and effort, diverts valuable resources from their real targets, focuses on the innocent, gives the public a false sense of security but worst of all, completes the terrorists job.
The problem with profiling of course is that it would give the likes of Al Qaeda an easy side door in. All they have to do is find a clean skinned operative, i.e. a European of fair complexion open to their ideology with no previous links, and they know he won't be screened at all. Even better if they can find an elderly, female European, going by some of the logic on here. No chance of her being screened, as long as we follow your line and just search the dark skinned men with beards eh?
I think it's easy to throw stones at the few individuals who underperform at central search (and we've all met one), but the anecdotes are thankfully the exceptions. I'd rather the majority keep doing it efficiently and give AQ a hard time trying to second guess when Bert Stuffup will be on duty than tell them that only a certain demographic will be screened.