There is only 3 reason why people scam. (1) some people scam because of their greediness about making money. (2) some scam because they have no job and they must feed their family. (3) some scam just because of their wicked habit.
I agree with all that you have said and it is pretty much annoying too see that people can have such terrible mindsets. However, do you know that in some instances, the number 1 and 2 is also related to people who allow themselves to get scammed.
Greed, which is looking for quick cash when they know in real life, it is not that easy, or having no job which if you do, you will definitely know the value of the cash you have. However, there are some scenarios though that may look pretty clean for someone to really know if it is a scam but the truth is if you ask questions, you will end up knowing.