Why the hell don’t they capture the methane and sell it?

It’s really hard. Most methane comes directly from the cow, and cows spend most of their time in air. The methane gets mixed in the air, in very small percentages. Extracting a small bit of methane from a lot of air is complex and energy Intensive, and methane is cheap.

So you’d spend a lot of money and power to produce very little money or power.