I'm by no means a savvy investor, nor an expert in this, and am probably stating the obvious but the rationale is Active Users have a value, even if they're not

I'm by no means a savvy investor, nor an expert in this, and am probably stating the obvious but the rationale is Active Users have a value, even if they're not actively providing revenue. Previously, it was simply monetizing that audience via Ads. More recently it's monetizing the data that those users generate. There is also value gaining a particular segment, such as Activison-Blizzard purchasing King gaining access to all the casual mobile gamers. Essentially, its that those users could be used to generate money, and thus are valuable.