it could be kind of disrespectful towards the culture that built them. I dont think we can restore them in such a way it respects the beliefs and culture of those who built them, so best thing to do would be to make sure they dont decay further. Also, restoring them just so tourists can have something to gawk at would be a kin to showing middle finger to those who built and revered them. And i doubt anyone would be interested in doing anything without monetary gain from it.