This is very basic mathematics. If they have to do other crime to get their hands on the cash it would cost for them to spam, then why spam at all instead of only focusing on the other, more profitable crime?
Spam by definition is mass messaging. If the price per message is higher than the expected return per message, it becomes pointless.
Spam is advertising for their other more profitable criminal enterprises. It's a means to an end, not an end itself. If the goal were simply to send messages, there would be no content.
And the efficiency of spam is so low, that with a high enough cost per message, it would not be a net gain for the spammers. If you pay a million dollars to send a million messages, but only gain $100 000 from the subsequent scams, then it's a bad venture.