I don't know what your life is like, but for me that's definitely the most likely explanation. When I'm not showering, meditating, or horizontal in bed, it usually means I'm reading something, watching something, working on something, or engaging actively in an internal monologue (that usually is more about reducing variables and maintaining control or the illusion of it, and less about creativity).
The moment my brain gets some free time, and there's nothing acute to worry about, ideas start to appear and I get excited.
Occam's Razor: The (simplest) explanation with the fewest assumptions tends to be the correct one.
It's almost always that simple.
Also, when you're travelling or on the train, and your wonder sense opens, you are relaxed. Less constriction to the arteries and blood vessels. As with lying horizontal, increased blood flow activates the brain.