I think the explanation "you’ll end up exhausting your own resources" is wrong for this case. I think you are correct that the bots are simply too dumb.
The likely explanation is that the bots are just curling the expensive URLs without a proper JavaScript engine to solve the challenge.
E.g. if I hack a bunch of routers around the world to act as my botnet, I probably wouldn't have enough storage to install Chrome or Selenium. The lightweight solution is just to use curl/wget (which may be pre-installed) or netcat/telnet.
The likely explanation is that the bots are just curling the expensive URLs without a proper JavaScript engine to solve the challenge.
E.g. if I hack a bunch of routers around the world to act as my botnet, I probably wouldn't have enough storage to install Chrome or Selenium. The lightweight solution is just to use curl/wget (which may be pre-installed) or netcat/telnet.