The resource implications are the problem. The cost - in terms of compute time - to bake those ads eats into the profit earned from advertising as a whole. Since only a fraction of users adblock, they would probably lose more money than they gained.
They'll consider it once the compute cost inserting the ads is low enough that it's worth it. I have no idea if we've reached that point yet, but I'm guessing not, since otherwise they'd already be doing it.