Originally Posted by
TheOOB
It's a balancing issue. One of the reasons casters were so broken in 3.x was that in addition to the fact that they got new, more powerful abilities every level, their old abilities got more powerful as well, so that by the time they reached double digits they had dozens of spells they could use, while other classes only had one or two abilities.
By making it so spells don't scale up, they have reduced that problem. A level 10 wizard won't have much use for their level 1 spells save beyond a little utility and as a back up when they run out(and the status inducing spells won't even work on foes by then). So while casters to get more and and more abilities, only their highest couple of spell levels will be truly relevant in most situations(but their lower levels spells are still there). I think it's a good balance, it's better than the alternative.