I think this is precisely why they (plan to) do it. If they placed a high enough emphasis on money over other job characteristics, they'd go to Wall Street, either with just a bachelors or a full-on finance Phd, depending on our discount rates. Grad students not only tend to have low enough discount rates such that their decision is optimal for them (but not irrational as the humorously Simpsons suggests), but I think they tend to value doing their passion very highly. (maximizing lifetime earnings doesn't map as well to maximizing lifetime utility for grad students, but it may for people who become bankers right after college.)
If they wanted security they'd become health care providers or some other job for which demand is long forecasted to exceed supply.
as far as accounting PhD's...I'm not sure...it takes a special type of personality to really like accounting, and I definitely don't have it. (I've only met a couple people who majored in it.) though I'm sure you'd make a killer CFO with an accounting PhD...