Can someone explain why you can attack someone over and over and win but you can't rob them???

Robbing works differently than fighting. In fighting, your Mafia size is determined by the number of people who are actually in your Mafia. In robbing, your Mafia size is determined by what level you are on. When you attack someone, it's based mainly on your attack and defense, but when robbing someone it's mainly based on your mafia size.

If the only property they have is Mafia Mikes and vacant land, those types of property can't be robbed.