I know they have team(s) of very smart people dedicated to solving this issue (at least at the individual level).
So assuming they care, I can think of two main reasons as to why it is not solved yet, both related to scale as Marques mentioned:
1.) Scale of the problem - It might be that they are already catching 99% of the stuff and we just see what falls through the cracks
2.) Scale of the solving - It could be that the teams and infrastructure are so large that they can't make the rapid adjustments needed compete in such an arms race
On a separate note, I imagine a higher quality comment section would increase engagement more than any "appealing" scam.
So assuming they care, I can think of two main reasons as to why it is not solved yet, both related to scale as Marques mentioned:
1.) Scale of the problem - It might be that they are already catching 99% of the stuff and we just see what falls through the cracks
2.) Scale of the solving - It could be that the teams and infrastructure are so large that they can't make the rapid adjustments needed compete in such an arms race
On a separate note, I imagine a higher quality comment section would increase engagement more than any "appealing" scam.