I’m not particularly criminally minded, but on the surface, it’s easy enough to think that you could make a return on activating games on a platform where each game can in theory make you a very small amount of money in trading cards, especially as there are programs that will idle a game for you until you get said cards.
Of course, once you have the cards, you are then eligible for booster packs that provide you with more cards to sell. And of course cards can be traded to level up your account, that in turn gives you an increased booster pack drop rate.
“But wait!” you say, “surely it’s not worth it, for such a tiny amount of money?” Well, good point, to start with I am sure it isn’t but as you continue to do it and make more accounts, (so that you can activate the same game multiple times) you are working towards a critical mass.
This Reddit post is a few years old, and since then Steam have taken some steps to prevent card farming, (like buying a game once/twice(?) a year), but it still lays down the theory behind what we are talking about.
I haven’t looked at the numbers closely and don’t intend to as we are just theorising here, but even if the numbers are way out, there is still the potential to earn money by doing nothing (except monitoring your bots and updating their scripts, which some of us think is fun anyway 
)
Looking at the thread, the main objection that people have to this person’s theory is that you still have to buy the games to activate which wipes out your profit margin - Well, that’s easily solved…
Interestingly, no-one in that thread seems to consider the idea of automation, it’s all about how to manage many steam accounts or activate games &c. As someone who looks at most things in life and tries to determine how a computer could help me or even better, do it for me, so I can do something else instead (probably look at something else to determine whether I can automate that &c.) this surprises me.
There are probably other markets too, selling the keys to other bot farmers that instantly activate them or something, really where there is a will there’s a criminal network and market place, as I said it’s not really something that I focus on or know much about, I’m far more interested in the ‘how’ you would do a thing technically, than ‘why’.
So much so that I did toy with the idea of writing a fully functioning bot that would monitor Chrono.gg and immediately message someone if they posted a steam key that could be ‘nabbed’, that would give me the interest of writing the code without the maliciousness of stealing stuff, I didn’t in the end as other more interesting prospects came up, it also occurred to me that @lonin et al. might not appreciate it, and I quite like hanging out here.
I’m happy to be proven wrong on some or all counts, but the fact of the matter is that bots farm keys, surely there is some infrastructure behind it, and if so, it must make money or why are people doing it?