People's mental model of the world doesn't include principal-agent problems[1]. People with actual equity in competing businesses tend to be pretty rivalrous (e.g. owners of small independent restaurants on the same street); naively, if you don't see employees as anything more than extensions of those rivalrous owners, you'd expect them to behave the same. But instead, of course, "it's just a job."

[1] https://en.wikipedia.org/wiki/Principal–agent_problem