The entire idea is that people hate being alone with their thoughts and will do nearly anything to avoid being in that situation.
I actually get comments from other passengers now how I can sit and look out the window for hours without music, movie or book.
In their rooms, they have other venues and probably less fear of negative self-conscious thoughts (too fat, ugly, whatever for the beach).
I wonder how many Hikikomori are in their own thoughts vs escaping into online venues.
I can think of half a dozen very personal conversations with random seatmates about their lives and problems. Something about opening up to strangers, and am pretty sure it wasn’t my personality or looks. Now that doesn’t happen anymore because you’re never trapped together with no outlets for hours on end.
One of my writing ideas (probably not original) was a retired psychologist who just flew on random flights and talked to whomever they sat next to. Now that plot would make no sense.
When you're finally just thinking about yourself, your "desirable surroundings" will disappear. You'll be trapped in the discomfort of your mind. You'll then try to escape these thoughts by focusing on the picnic, sauna, beach.
Sitting in a room alone is just a metaphor for only thinking about your inner self. You can do that anywhere. And most people can't stand it, so they do something else. So, your conclusion is right, in that people would always prefer to do anything else other than focus on their inner thoughts. Like being in a place that distracts them.
The key thing, I would claim, from a "whole person" perspective, is that the contrast is "sit quietly, forget your body entirely and think" and "get stressed by bodily tension and start an uncontrollable argument with yourself" where "bring your mind and body into harmony through movement" isn't an options.
However the OP said "an indoor room isn’t a particularly desirable environment" - and that actually is absurd. If you need to be in a sauna or on a beach then you're already too distracted by the environment to be able to think deeply.
Of course there’s nothing wrong with doing things, but there is, perhaps, with always having to be doing something.