but the culture here wants to believe there is some huge advantage to switching jobs every 30 months or so.
There is: your salary goes up, and in general is maximized according to what your value on the open market is. Companies won't give decent raises if you stay with them long-term, but by getting a new job you reset your salary to what the current market rate is. The downside to this, of course, is lack of stability for the employee, but it does help keep you from getting stuck in a rut.
Now how this situation benefits employers, I'm not sure. There's nothing stopping them from offering better raises and retaining employees so they don't have to suffer the costs and effects of employee turnover, but they just don't want to do it for some reason. I guess they're hoping that some fraction of employees will be averse to spending effort job-hopping and dealing with the uncertainty that comes with that, and that they'll profit that way, but what really happens is the worst employees are the ones who stick around for the reliable paycheck, and the best employees jump ship for a better offer in 18 months.