Perhaps the question should be asked why the building was placed close enough to the taxiway, so that it was possible for a large jet to clip it? The taxiway was there first.
If it is any consolation to the BA pilots, they are the fourth aircraft to have made this mistake.
It is noticeable that this is a sensitive subject amongst the BA flight crew. They have been a cocky lot in the recent past, and now a little humility is being displayed for a change.