Pickled Soul said:
Simply enough - margins.
For example I can create a quick and simple spambot to send a million emails a day, and all it'll cost me is the electricity to power my laptop (and not even that if I've slaved someone else's system), and even if only 0.000001% of those people buy something that'll be one person per day for zero cost.
Now give me a botnet and I could be sending a 100 million emails per day with the 0.000001% take up rate.
No other marketing strategy has those numbers for that cost (zero!), and yes people do actually buy things from all these viagra / breast enhancement spam emails.
So with the forum spambots, minute take up percentage but again zero cost, if even one person in a million buys something they've succeeded.
I think the profits are exaggerated, it's like the telemarketing calls in the first days of the telephone - at first it made large profits out of the cheap technology that allowed them to connect to peoples homes.
However precisely because the system was so cheap to utilize, the market quickly became over-saturated and invasive in it's practises for attention so that the target audiences stooped paying the needed attention and stopped purchasing from telemarketing calls!
Why does it still exist - because it's cheap and creates the illusion of actual tangible profits precisely because of the low costs. But the more it's used the less effective it has become.
Same with spam bots, people have wised up on the spam adverts and scams, we don't pay attention to them any more!
It is used because it is virtually free and limitless in its reach, but it only gives the illusion of possible profits.
Spam does not work because there's a limit to how much information you can soak when bombarded with ads.
The reasoning of some people that it must work because it's being done is called affirming the consequent, it's a logical fallacy that we're prone to fall for.
And if you think that people aren't as stupid as to keep using a tool even after it's no longer useful, just think about the phone books that we keep getting or the 20% of Internet user who still used Internet Explorer 6 (10 year old browser!) last year.