Some Duelists do not attack because they are worried about a Flip Effect on a face-downmonster. But if you do not attack, then your opponent will just Flip Summon the monster on this/her next turn anyway. The monster is not going to go away just because you do not attack it. You might as well attack and get it out of the way.
The same is true for Trap Cards. Unless you think you might draw a card that will destroy Trap Cards on your next turn, you might as well attack and get it over with. Waiting just gives your opponent a chance to destroy your monster or Set more Trap Cards.