Unity: We have to charge for every install because we only see totals. Also Unity: We can tell which install is which, so you won’t be overcharged.
Because they realize that a huge number of their customers are small indies, and they want to be able to squeeze them - the majority of their customer base - not just the minority of big companies (who are also the most likely to fight back legally).
Just look at how their scheme squeezes smaller, poorer developers way more than big ones. If Unity went by points like, say Epic does with Unreal, they could shake down the big developers… but wouldn’t get much out of the indies.