I think it's rather the opposite - there's way too much going on in their life, specifically stuff that they have no control over, so they vent all that stress wherever they can.
You could say the same about most Internet activity: busy people don't have time to post on HN, or make stupid LinkedIn posts. Yet here we all are, reading and writing despite our busy startup lives.
Oh, my bad, I should've phrased it differently. I didn't mean that they're necessarily busy and have to handle a lot of matters, but rather that a lot of things are happening around them. It surely can be stressful even if one's not actively involved in something, but if they're merely witnessing something happening.