This complaint is tantamount to me saying: I spent $20,000 advertising my new cloud provider on twitter, 5000 people signed up for an account but only 5 of them ever logged in so only 5 converted. That isn't how it works.
In google ads a conversion is clearly defined as the users clicks the link and then completed a second action (pre-register in this instance). When you create a campaign, you define the goal like this: when a user clicks the link and pushes the pre-registration button the user goal of conversion is complete.
Just because you don't like what happened thereafter isn't the concern of the ad platform.
To take this even further, the install isn't what matters either, it's the user buying an in-app purchase, or it's the user still being subscribed 1 year later, or it's the user buying high margin merchandise after 3 years as a loyal customer...
Every part is a step in the funnel, and every step down the funnel is harder and slower to analyse. Ad marketplaces only have the data of the very first step most of the time.
Doing this analysis is The Job. That's all of what marketing (particularly digital marketing/PPC) is, and being good at this and building the loop is the difference between buying clicks from people who aren't actually that interested, and buying clicks from people who will pay for a product.
