science

Good question, apparently there's stuff we understand and stuff we don't.

Purely a guess on my part, but I imagine a not-insignificant amount of this is simply bloodflow. If cancer is a disruption in the cellular replication mechanics, then we're looking at protein folding, DNA replication, and ion pumps and cell stuff. Perhaps not "taking out the trash" via blood flow allows higher rates of cellular corruption? Maybe lower bloodflow results in weaker cell self-destructs? Lots of candidates...

There's one paradoxical-seeming fact that adult humans generally use the same amount of calories each day regardless of their physical activity level. There are exceptions, but they are extreme cases, like professional athletes pushing their limits.

It makes sense if you think about it. Processes that give us access to energy aren't exactly fast, so our bodies will just be producing that energy regardless so it's there if we need it. But when we don't, any cells can grab them up to do their cell things, which includes cancer cells (which we all have but are mostly benign), giving them more chances to make more mistakes copying DNA, which rolls the dice more often on picking up some of those mutations that make them dangerous or deadly.

It's also why you lose weight in the kitchen rather than the gym. Your body isn't doing anything differently with the food you eat whether you take a nap or go for a run.