It doesn't make sense if you are not rich.

Completing a PhD typically takes 5-7 years in the US. In my public university, the nominal tuition for that time would be $100-150k for in-state students and $180-250k for others. Then add living costs on top of that. A PhD increases expected lifetime earnings over bachelor's, but not in all fields and definitely not enough to justify such spending.

That is how it works. PhD programs charge tuition. Tuition is typically reimbursed through some working arrangement, but you're welcome to pay out of pocket.

Because a PhD should be thought of a job, not pure education. PhD students are already underpaid, go over a lot of stress, and now some wants them to pay for these? Doesn’t add up at all.