Are antibiotics prescribed because white blood cells are not enough to fight infection ?

Yes and no. For most bacterial infections, antibiotics will be prescribed regardless of whether or not your own immune system would be able to fight it. For example, you may be able to fight a strep throat infection on your own, however you may not depending on many other factors and it may spread and become much more severe. Same with an infected wound, it's better to treat it with antibiotics right off the bat.