Why does Shen have AP scaling?

Is Shen suppose to be played as an AP champion, so you can do more damage to other champions in late game? It makes no sense why Shen had to use energy, yet doesn't scale throughout the game. I personally think riot should remove AP ratio on Shen since it's useless or replace it with AD or HP scaling, but has a weak scaling. This makes Shen slightly stronger in the late game, so you can do damage to other tanks. Since other tanks get MR and Armour which makes Shen more useless throughout the game. Another idea that i had came up with, was to make Shen W applied to his ult. So when Shen ult someone, that person would have a W effect applied to them, or make the ally have half of Shen's MR and armour when under the effect of Shen's ult.

I personally love playing Shen, but hate the fact that he doesn't scale like every other champion.