One mod wasn't sufficient to keep bots at bay on a podunk PHP forum with hundreds of members 15yr ago. It's not sufficient to stem the tide of far, far better bots that lurk the internet today. There's other mechanisms at play.
And they are…?
So basically it's a tough nut to crack for little gain. Why bother when you can write Reddit bots that are far more impactful and will earn far more money.
Edit: Yes I know stuff is recommended to be bought or sold here. It's absolutely nothing compared to Reddit where you can pimp viral content for a brand as top level content and where you have subs that are specifically about consumerists things (what X should I buy type subs).
There are plenty of direct and indirect recommendations for goods and services. "Who's hiring" is a good example of that. Posts about a particular product often garner posts about similar products. To be clear, I agree it's not the main thrust of this site, but let's not be blind to it happening, often for purely benign purpose.
We do also write software, so the mod work is (thankfully) not all manual.