Personally, I just find it hard to be on a bike for more than about 50 miles. As much as I like cycling, there is a tipping point where I just don't want to sit on a bike anymore.
It's not so much that I'm just physically spent...I just feel too uncomfortable to care anymore.
To me it's a hobby and a way to keep in shape, and to also socialize with other cyclists. But it comes to a point where "doing something else" overrides my desire to sit on the saddle any longer.
But that's me.