Many players like to feel like their characters have progressed in power.

If it takes just as much effort to kill something at level 90 as it did at level 50, what was the point of leveling up? Just to get a couple of extra buttons to press that make you no more effective than you were 40 levels ago?

Also as others have pointed out, scaling content just to level cap isn't straightforward and very time consuming when it comes to testing. Now imagine that everything had to be scaled to every level. You can't scale everything to level 100 because not every character has all their jobs to 100. In order for them to do the content, it would have to be scaled to all the intermediate levels as well.