The reason that people don't switch jobs is that there's currently no way to get a degree other than by choosing to start with one.
As a result, you just pick the job with the highest starting salary and go from there because spending all the money for the qualifications at the college isn't really worth it unless you're already so rich that you can blow away thousands of dollars and not care (which defeats the purpose of the job system to begin with).
If it weren't for that, I might at least consider say, getting a temporary job while in university to get a degree and then switching to that degree's job.
Plus of COURSE peoole are going to stay at the same job because they wanna fatten the girls there over time and read the dialogue lines of the ladies commenting on eachother's weight.