There are other reasons not to do things on-device. For instance, being able to pick your own ads might hinder the ability to defend against ad-viewing bots. Al

There are other reasons not to do things on-device. For instance, being able to pick your own ads might hinder the ability to defend against ad-viewing bots. Also, traditional ad-tech also has to deal with meeting global target spend rates (e.g. if client A pays for 1000 impressions, and client B pays for 9000 impressions, then 90% of the ads must be for client B even if every device would prefer ads for client A). Usually there are realtime databases keeping track of impression counts for all the campaigns and it would be infeasible to synchronize that with every device so that they could make local targeting decisions. I.e. you can't necessarily make the decision locally if it's not local problem, and ad-targeting usually isn't a local problem.