"Why don't more people walk out of interviews?"
They freak out. I got blacklisted at a cellphone company for doing that a couple decades ago. I found that out via a friend on the inside. Basically I was looking for, and applied to get a lab bench job (basically a spectrum analyzer / communication analyzer jockey, beneath my ability, but it was an exciting growing company etc) and they wanted me to be a junior roving field tech, basically I'd help the real field techs carry heavy stuff into the building and then (literally) mow the lawn while the real tech worked. And I went to school for X years to run a lawnmower... uh huh...
If you think its mortifying for the candidate, imagine how the HR rep looks when you walk out, thus making a fool out of them. Note that I wasn't sarcastic or caustic or anything when I left, after all, I had lots of friends / acquaintances on the inside and telecom is a very small world. Lots of hand shaking and "you have a nice facility here but I'm not interested in the position" and so on. I got word from my inside contacts that the HR girl was fuming with rage and swore after I made a fool of her by walking out, that my resume would never pass her desk again in any capacity, etc.
Companies are always doing stupid stuff, just hopefully not too often, and they're at least partially interested in how you'll respond. So imagine you work there and someone in upper management makes the worlds dumbest presentation to the division, they're terrified you're going to laugh at them and walk out.
When I was younger and (even more) stupider, stuck at an interview I decided I didn't really want, I started flirting with the attractive HR staffer, which didn't work out, but it was fun at the time. Times were different back then, now a days politely asking a woman out at work would probably get you fired or arrested.