Correct - because the cost of the spam itself is negligible, but the cost of developing a prospective mark into cash-in-pocket is quite high in terms of time & resources.
The last thing the scammer wants is for the mark to back out after the scammer has invested resources into them, and before they have realised any gains.
Effectively, insufficiently gullible marks reduce their profit margins.
I would like an email service where, when someone wants to send me an email, they have to first get some hash, e.g., "{hash}.myaccount@myprovider.com".
Then, in order to get the hash, they have to paypal me 20 cents.
If my attention is not worth 20 cents to them, then they should not be contacting me in the first place.
Further replies to the same thread should be free for some period of time (e.g. a month or so).
The last thing the scammer wants is for the mark to back out after the scammer has invested resources into them, and before they have realised any gains.
Effectively, insufficiently gullible marks reduce their profit margins.