I can't speak for IndianAstronaut, but I (PhD Physics) give the same advice.
My reasoning is that to actually survive and succeed in grad school, you need to be absolutely passionate about what you're going to be doing. If you want to go to grad school because it seems like the next logical step, then I give that advice to emphasize that it really isn't, and that it's a bad choice for many people. But if you really are enthusiastic about your field, and really want to keep doing it, then you're going to ignore all advice anyway.
The job market is absolutely terrible for PhD grads. Especially when you compare it with the fact that you are losing valuable work experience during that time frame. Going down the postdoc and tenure track is like trying to become the next Hollywood star.
My reasoning is that to actually survive and succeed in grad school, you need to be absolutely passionate about what you're going to be doing. If you want to go to grad school because it seems like the next logical step, then I give that advice to emphasize that it really isn't, and that it's a bad choice for many people. But if you really are enthusiastic about your field, and really want to keep doing it, then you're going to ignore all advice anyway.