Because the only metric that matters for social media is engagement, and a war against active account, no matter the purpose, would make that line go down.
Also, shill accounts prove to advertisers that the platform works to make money. That’s the whole reason Reddit exists.
YCombinator is not a social company. They actually do a ton to try to stop this stuff, it's just hard.
> Because the only metric that matters for social media is engagement
True, but what if a third party existed to measure and compare other metrics that users would actually care about across popular platforms?