The Fat el Hombre said:They're only human
Yes, but you'd expect a defender, when dealing with a cross, whether individually or as a unit, to be concentrating. It is their job and (on the whole) they are not doing it intensively for 45 mins.
Saying that, the keeper does have the best all round view of an attack, so it is his duty to organise his 'unit', if he sees potential danger.