You're right - the game is easy but as other people have said the real challenege is in the extras. Read: monster arena, dark aeons + penance.

In FF7 you could get KOTR and kill sephiroth in 1 move. You could also level up and then defeat Emerald weapon and Ruby weapon in a few minutes without dying and without even trying. It's a piece of cake.

Try fighting Penance though and you'll be in a battle that lasts a good hour. That's when you've already spent 130 hours or more levelling up so that you're hard enough to fight him.

It's the same in all FF games - the main storyline fighting is easy - but the extras can be as tough as you want.