They chose not to because it would lock most advertisers out. That's pretty much counter to everyone's goals. It's analogous to the "stop crime by jailing all of Texas" solution.
By the way, this is already done since Google does check advertiser accounts against various sanctions and watchlists as part of dosing business in every country they operate in.
The point is that it’s not a resource or scale issue (as you keep arguing), but a profit and incentives issue as I said before.