There is a misconception. Nobody (but researchers) earn money building bots. Bots are a way to achieve something but bots alone are not good to profit. Even those guys that sell bot on BHW are not really earning a lot of money but spare change.
Bots work in tandem with a white/black hat technique.
For example, let's say you spend a couple of months working on a bot and you finish it (and it works). So then what?. If you survey some bots sold on BHW, then some "successful" seller are sold around 100 or 200 (there are some exception but the numbers are really thin). Now, let's say 100 x $20 per bot (some bot could cost a lot but it is for an expert) = $2000 USD per two months of work, or $1000 USD per month. It is crap of money,
there is a lot of ways to earn better money programming).
Now, designing a proper bot takes more than a couple of months. There is a difference between a lousy automate script (that everybody could do) and a proper bot and is the research of a vulnerability.