There is a reason they didn't. They fear the US government's reaction.
Edit: Why downvote? Do you really think that the US government will stay silent if the European Union threatens with such fines? Political tensions are something you take heavily into account.
I'm _really_ tempted to write that they could use the fine to finance the information campaign, but I know that government finances doesn't work that way.
Governments are likely walking a much finer line than we might imagine. Imagine they carried out your idea. The EU is a political organization manned by a large number of mostly professional politicians. Google is world's largest data harvesting and advertising company whose products are used, on a daily basis, by a pretty sizable chunk of our entire species' population. Imagine if Google decided to fight back. Who would be able to create a more effective "information campaign"?
I can't help but consider one current "information campaign" in the UK. In response to skyrocketing violent crime they've chose to put anti-stabbing/knife messaging on fried chicken boxes, Literally [1]. Knife amnesty bins and fried chicken anti-knife messaging. That's a government "information campaign" through and through.
Consider Brexit for a minute. The most recent polls show support for leaving, but throughout it's been extremely close. However multinational corporations are universally against Brexit - a global world is a more profitable world. And these same multinational corporations tend to have a strangle hold on the places most people get their news from. This can be from the news agencies themselves (Disney owns ABC, Comcast owns NBC, Time Warner owns CNN, etc) but more directly also from the way that people get their news. For most people that is Facebook and Google. And these corporations tend to promote what is their own best interest. As a specific example CNN ends up being chosen for about 20% of Google's news recommendations. It's a deeply partisan site that's not uniquely popular and has a dubious track record when it comes to reliability. But their agenda and Google's agenda fit nicely.
Consider the two topics above, combined. The global media has nearly universally tried to condemn Brexit. And while media clearly doesn't have as large as an effect as some would like to imagine (Facebook was seeing an exodus of young users before any media outrage - it's become the social media site for your mom), it equally clearly does have at least some effect. And so imagine Google simply swapped their bias. And was suddenly now disproportionately promoting messaging come propaganda against the EU, in favor of Brexit, promoting things such as the yellow vests in France, the various leave campaigns gaining momentum in other nations, etc.
When topics, even with the media disproportionately on one side, are so close - if that media that people were presented suddenly started lobbying for the other side, that would have a massive effect. I don't think it's hyperbolic to suggest that companies such as Google and Facebook could effectively cause the EU to collapse if they so desired. It's already on somewhat shaky ground with near universal media support. If they don't play ball with the companies that direct that media, that ground very much stands to give way.
That doesn't mean the governments are completely obsequious to the corporations, yet, but it does mean that the corporations are also in no way obsequious to the governments. And I think this interbalanced relationship is one major reason that we see increasingly see governments reluctant to do anything that could meaningfully negatively affect mega corporations or other very powerful players. It's also why I see us gradually headed towards more overt corporatism. Corporations grow exponentially more powerful by the decade, and this shows no signs of abating.
GDPR enforcement is 15 months old and regulators aren't the fastest bunch. They're also cooperation based as the goal isn't to serve fines but to ensure compliance: If you cooperate, you might get away with no fine at all (depending on circumstances).
Also the 4% global revenue fine wasn't exercised yet because it's the maximum fine, and there needs to be room for escalation: hard to serve a bigger punch if you're already at the maximum.
