The real question is: why does Bitcointalk give a flying fuck about bounty spammers?
It's allowed to spam millions of links to spam on social media.
And as long as they don't pay something that has real value, but only in tokens they made up by themselves, spamming has no real cost and only benefits to them.
I understand that the question is rhetorical, as it seems to me that this has already been discussed many times, and I think it is because management prefers that this traffic stays here rather than go elsewhere.
If it were as simple as if they go elsewhere, the rest of the traffic remains the same and therefore there is more net quality in the forum, it would be desirable, but you do not know if it will reduce the overall traffic of the forum even more.