There's certainly no financial incentive for a Ph.D. graduate to remain in academia. That said, there is no financial incentive to do a Ph.D. in the first place. My understanding is that the opportunity cost (salary you didn't earn while doing your Ph.D.) is larger than the total expected extra earnings over your lifetime. My understanding is that by contrast a Masters degree is a good investment.

There are other reasons to stay in academia --- mostly to do with lifestyle. You have a lot more freedom to work on whatever interests you than in many other career paths.