Honestly, I think you're asking the wrong question, and because of that you're getting answers that are relevant to slightly different aspects of the overall conversation. A more apt question might be, "why are so many Personal range spells so damn good?" But that question may not have any good answer.
It's not that the developers were deviously trying to make casters rock harder at melee than Fighters, it's just that they were pretty dumb and just did whatever felt right without testing anything. I see a lot of posters here basically saying that Personal range is just retarded in general, which is a stupid premise. There are many reasons why a spell should be designed personal-only, not the least of which simply being, "because it feels right." And I bet that's the exact reason why spells like Mage Armor and Righteous Might are personal-only, because it feels right. The problem is not with Personal range in general, the problem is that some Personal range spells are just really powerful. Too powerful.
In fact, maybe the worst justification for a Personal range spell being personal-only is, "because making it usable on others would make it 'too powerful.'" Because that brings us in large part to the large problem we have with spells like Divine Power or Mirror Image. In my opinion, Mirror Image is simply too powerful to be a 2nd level spell and there's no reason for it to be personal-only other than "well, it's really fucking powerful, let's make it personal-only." So, how about making it a 3rd level spell and allowing the caster to create one Mirror Image per caster level (or two caster levels, or whatever), each of any creature within some given range, to a max of 8 per creature? Not only does that make the spell much more interesting, it allows the caster to use it on others and it puts it at a more reasonable spell level. It could probably even be a 4th level spell with that effect.