There are methane digester toilets. Some of them are even well-integrated with buildings.
But even between humans and all lifestock there isn't enough bullshit to fill up the American demand level. And methane wouldn't supply the broad range of hydrocarbons that crude oil supplies for industry.
If we were intelligent and adaptive and forward-thinking, we would retool our industries to run on solar and wind, with biogas to shore up the base rate and fill demand spikes.