As many others have already stated, escapism.
When I'm out on the bike, it's my time, which tends to be fairly limited in busy life. No one to annoy me, no problems and issues to deal with. Just me, the bike and the road/trail.
Also, not trying to be an eco-warrior, but with all the complaints being made about clean air and pollution (generally by hypocrites) in the UK. There is a moral satisfaction about not contributing towards it.