Most of the world's computers (and routers and toasters) are 99% idle and unsupervised. So the spare space and cycles are repurposed by malware. Converting space and cycles to cash is straightforward. You'd think malware has to be fairly stealthy to avoid users noticing and reimaging their machines, but no, it's quite difficult to interfere so blatantly that "owners" step in. Other malware is the real competition. So it's competing bands of malware developers, all using it for boring cash generation. Other bands steal cash directly. And underfunded governments occasionally step on some toes. A few bands pursue more interesting goals.