They think this for the same reasons all employers think this; employment is a watered down form of ownership.
Advancement up the social hierarchy allows one to abuse the people below them. As near as I can tell from my own experience: money and control are the things you receive as rewards.
A top neuro program in a major city will pay ~40K per year (and this is high relative to many other sciences). The rest of the compensation is health insurance and a meaningless "tuition" credit in order to register for a full "courseload" of research. Even at 40 hours per week that is shit compensation relative to what most of the students could be making elsewhere. The thesis project takes place over ~5 years, which is more than enough time to produce very good output on normal working hours. If students want to make their project their whole life fine, but as it stands there is way too much pressure coming from the top to do so, with minimal real benefits actually reaching the students.
It's also supervisor-dependent! After I was formally accepted into a post-grad program, I was talking to my supervisor, my then-recent engagement slipped into the conversation and h - a senior bachelor and resident grump - did not approve of me getting married. He thought it would be distraction; I very much doubt that it was departmental policy, but my future research hinged on his whims.
Luckily they managed to avoid the specific IP in the papers that the university owned, and start the startup anyway. It has gone on to be a successful business.
Obviously, the university lost out in every conceivable way from this scenario. Literally any other course of action would have given them a better result. Play stupid games, etc.
edit: this was Australia btw