Making advertising more efficient would also open up opportunities for smaller players. Right now only the big guys have the chops to carpet-bomb the market regardless of bots. Noise benefits those who can afford to stand above it.
Pay per impression is a metric adexchanges use. So it is not in best interest of companies to remove bots. Bots create "economic activity" when such metrics are used.
A lot more. Preventing bots from eating up your entire digital advertising budget takes a lot of time and money.