Think of the AP cost as both the firing and the reloading in one attack action. No difference.
I get that, but a crossbow you'd keep loaded and you could fire once and draw a melee weapon at a very small AP cost. A bow would have a higher AP cost for something like that.
Or fire and duck behind cover to reload. Again, something that would be easier with a crossbow. You could spend the high AP reload in safety. Particularly on heavy crossbows with a windlass.
Simplifications. It would also give the bow and crossbow very different use profiles vs just how many times can you fire a round with your AP. I think it'd be interesting.