Ad auctions have 100ms to reply to an auction request.[0] This does not give you a lot of time to display the perfect ad for a given customer or search. Especially when you need to do this at the volume that Google shoes ads.
That's unlikely how it actually works on Google's site.
Google doesn't have to connect to an advertiser's network, they already have the ad information and all targeting and bidding information, so they can mostly compute the results ahead of time and only do last minute checks for winners based on, for example, whether the most likely winners have exceeded daily limits and such.
[0] https://www.quora.com/How-do-ad-exchanges-and-real-time-bidd...