Take a person that has pirated many games, how do you know which ones they would have actually paid for? Even if you knew the ones they play the most, it woulnd't necessarily translate to a buy in the same order.
You can also estimate sales within a 5% margin of error. Other factors such as play time and ratings also allow you to understand what sales would have been.
There’s lots of data to draw from.
Is what? Pirated? How can you have concrete data of something that didn't happen? A copy that didn't sell can not count towards sales.
If it is based on sales estimation, then it could simply mean that the game didn't sell well because it was bad. Not every unsold copy of an estimation is automatically a pirated copy.
Keygens tend to be based on the cracker reverse engineering the licence key check algorithm, and then just spitting out a bunch of keys that fit it. Most people will pick the first key the keygen creates, which makes it easy to track the pirated copies.
The only way around the wholesale pirating of games is to make the game totally nerfed unless it can connect to a 'server'. This is why most AAA games all have at least some level of online features.
A release which gained 10M in sales which could not be pirated and scored 80/8 on metacritic can be compared to a release which sold 4 million but scored the same and has the same play time curves.
Obviously this is contrived, there are way more inputs, but you can see where I’m going.
Maybe the full data is more convincing, but ultimately you can't really know whether someone playing your game would've bought it other than if they did. If, for example, each iteration of some franchise sells less with more pirating going on, it probably means your fans don't appreciate the way things developed and just want to see where their once beloved franchise is going. Assassin's Creed used to be one of the best in gaming, but for me it's not even worth pirating any more. Should've developed the current-year story like it was foreshadowed in AS2, but the whole idea of training Miles using the animus to defeat abstergo was jus dropped to milk the fans once a year. Doesn't surprise me when people stop buying in such a case.
I mean, tons of people on this page are saying DRM doesn't reduce piracy and serves no purpose. But it's hard to make that position jive with the fact that the companies with tons of data on how DRM affects their bottom line keep paying for it.
I have actual concrete data about this¹ and unfortunately the percentage is much larger than you realise.
I'm still under NDA until 2024 but it's in the region of 60-80% of sales for certain games, especially RPGs which seem to get pirated the most.
People like to cite hugely successful products being DRM-Free or something as proof that it works, but for every project that works well there's hundreds that die in obscurity.
Also, people like to back a champion to "prove that it works" which inflates the margins of the champion product.
¹ - based on my time at Ubisoft, and based on Ubisoft titles only and when accounting for other differences between franchised games.