Look at it this way - the data of EU people is valuable only because someone eventually will get money from those EU people.
Showing targeted ads to EU people is only valuable because someone somewhere expects to sell stuff to EU people based on those ads; behavior data about EU people is only valuable because someone somewhere expects to use that data to get money from those people.
The end users (potential buyers) of that data will need to be GDPR compliant, because they'll be trading with EU and thus have a presence in EU - and they will have to show that they got all that data in a compliant manner and had consent for that. All the major advertisers with all their money won't be able to legally buy or use "tainted" data for which they have no GDPR-style consent, so they won't.
So if USAdCo has no presence in EU, and has been "paid with data", then it's been paid in a worthless currency - EU companies won't/can't buy that data, worldwide companies like CocaCola or Netflix or Amazon or other USA companies who have EU customers won't/can't buy that data since they have EU presence and will need to be compliant, and USA companies who don't have EU customers won't buy that data since it's not valuable to them.
What good is all that data if you can't really use or sell it? It's not enough for you to be out of GDPR reach, you also need your main partners to be out of GDPR reach; since if you're trading information that might contain private data, and they want to be compliant, guess what - they'll require you to be compliant as well, since they can't simply say "oh, they sold us that data, it's their problem", they're fully responsible.