In fact, that is not a silly question. They were aware of risks and gambled on how the enemy would react. Don't get caught up second guessing or armchair quarterbacking, you weren't there at the time.
The plan and the beaches that were selected was the best bet for the invasion with the information the planners had at the time.
The reason why several beaches where hit instead of one because the last thing you want to do is bottle neck your force in one area. This would allow the enemy to slow down and slaughter your forces.
