Maybe the problem is that they don't think this is true. How will it put them out of business? Lawsuits? Government forcing them to pull the plug? People not using the platform because fake stuff is there? I'm not convinced that the end result of any one of these paths will be strong enough to wreck their businesses.
If people realize that not many users are actively using these platforms it would decrease the value of ad buys.
I mean if 20% of all Twitter users are bots that would be detrimental to the sales department. If Twitter knew how many users are bots and did nothing it might be considered a form of fraud and deceiving investors.
Why should I buy ads on a platform that are overpriced if the ads will not be seen by legitimate users?
I think legitimate users will see them, but so will bots or fake users. I think there is a lot of click fraud, and has been since the early days of ppc. But we're still here.
Now, I can't really answer your question, which was "Why" should you still buy ads on the platforms? I for one don't. But many, many people do. Just like they buy ads on TV knowing a lot of people are going to ffw through them.
Maybe the problem is that they don't think this is true. How will it put them out of business? Lawsuits? Government forcing them to pull the plug? People not using the platform because fake stuff is there? I'm not convinced that the end result of any one of these paths will be strong enough to wreck their businesses.