("But but but look at all of my smart grad student friends getting their PhDs! They can't all be doing something irrational, can they?!")
Those interested in cash will grow tired of the funding cycles and move on.
There isn't much of a financial reward in research. You believe you're on the brink of discovering something others have tried and failed to explain.
The funding cycles will also make the people not in it for the money tired, except for the ones that keep winning; it's a rat race regardless of your interest in cash. Most research is not about being "on the brink of discovering something [...]"; that is a romantic idea of research which is exceedingly rare.
Money in the bank is not the end goal for everyone. You can work a job to get paid so that one day you don't have to work anymore. Or, you can work a job that pays sufficiently and you'll happily do until your last days of life.
Lastly, most research is on the brink of discovering something. Whether or not there is a life changing impact (or application whatsoever) as a result of that discovery is another argument all together.
Just that with most humanly shaped goals, the formula would be too complicated to be tractable, especially since you have to account for all your guesses of the unknown. People don't even know what makes them happy before they live through it, and you can only live through so many experiences.
So we `give up' and rely on rules-of-thumb and call them wisdom. And mostly we should!
Academia's contribution to that position is a murky proposition (added debt without added earnings potential undermines financial security). And while there are certainly immediate rewards from the pursuit of intellectual goals and from your friends at university, you can also get not-too-dissimilar rewards and more cash doing something like programming computers. And then you can use the cash to do things like: honeymoon with your wife in South America, enroll your child in a really good arts or music program, or spoil your grandchildren with a study-abroad semester in Europe.
If that's what you're looking for, you shouldn't even be considering a phd.
If I could live my life over, I would never have gone to grad school. And I'd have probably had kids 10 years earlier than I did.
- more overlap of your lifetime with the kids lifetime (so, see more of your kids) - get the scary parts over earlier, and enjoy more of the rest - if female, easier time conceiving
Or maybe that's just parenting in general.
Let's put some numbers on that. My first child was born when I was 35. I turn 48 this year, and will not have all my children in college until I'm nearly 60. I'm not looking forward to figuring out college when I'd like to retire.
Moving on, my family health history sucks - I do not have a male relative who has reached 60 without a heart attack and/or stroke. That includes 2 brothers that I know of. I'm working to take care of myself with diet+exercise, but already have hypertension. If my children have children as late as I did, there won't be a grandchild until I'm in my 70s. I have very few male relatives who survived to 70, and none arrived there with their minds intact. There is reason to believe that I could do better, but realistically there is a good chance that I won't.
I was aware of most of these facts in my 20s. They just didn't seem relevant to the life choices that I was making then. In retrospect, I wish I'd chosen differently. I won't push my children towards any particular life decisions, but I hope to give them tools to make choices that better reflect what they want in the long term than I did.
Fundamentally, as individuals, time is all we have. Using it wisely is important.