Are you allowed to attack creatures instead of a player?
Since this is a basic rules question, a quote from the basic rules:


  In your combat phase, you choose which of your creatures will attack, and you 
  choose who or what they will attack. Each one can attack your opponent or one of your opponent’s planeswalkers, but not any of his or her creatures.


You say it'd be easier to just attack creatures. Sure - and that's kind of the point. If you could just use a bigger creature to kill off a smaller creature, combat wouldn't be nearly so interesting, and the game would lose a lot. You really do have to work out how to manage to attack your opponent and get through without losing too many creatures or leaving yourself vulnerable. It's more work, but it's part of what makes the game interesting and challenging, and gives you an edge if you're better at it than your opponent.