This is sorta a self-correcting problem because bot traffic is imputed into the price/bids for ads. Bot traffic lowers ROI, hence lower bid. If ads converted really well and were 100% human traffic, they would be much more expensive on a CPC or CPM basis than they already are (even though they are plenty expensive still). Evidently in spite of bots, companies still derive enough value to keep spending tons of money on ads and pay really high CPM and CPC rates.