What's so special about specifically the PhD student experience that isn't accessible once you have the PhD?

My experience has been that research became much more fulfilling after finishing my PhD. I got more research independence, the level of work I was expected to do increased, and as a bonus, my salary almost tripled. It was like having the world open up, and starting to really experience being a scientist without my PI protecting me.

I was curious about their decisions because if you're taking on the opportunity cost of a PhD, it's probably because you enjoy research, but if you enjoy research, you wouldn't keep going back to the starting point. So, without additional context, it seemed like they just wanted the credentials.

I think it was also worth asking because universities often want to know why you want another PhD, since from their perspective, spending that funding on someone with no PhD potentially creates a new researcher (vs spending it on an existing researcher). So, if they managed to get into a PhD program again, they probably had a good reason.

Their response about different countries is an explanation (especially from an immigration angle), it's not like I'm asking them to lay out all their personal circumstances behind the decision in detail.