The short answer is: it's science.
Research across the board shows that good weightlifting behavior includes both high rep counts with low weight and low rep counts with heavy weight. Seriously - ask any personal trainer or casual bodybuilder you know.
This approach means that a great set can include 3 reps at high weight or 30 reps at low weight. What matters is that the set brings you to the point of exhaustion.
If we were to reward the total number of reps (which players could do at much lighter weight to earn more points), it would move people away from sets with fewer reps at a heavier weight, which are proven to be excellent for strength building.
Counting sets means you earn points for either kind of focus in your exercise - strength or endurance.
You can learn more about how we define a set here.