Ah, that's interesting. Different countries can be a fair reason I suppose.

Fair question. In my case, each PhD opened a door that didn't exist from the previous position. The mathematics PhD in Italy didn't give me access to computational chemistry labs in the US. The quantum chemistry PhD didn't give me access to materials science groups. Immigration, funding structures, and departmental boundaries created the path — not a desire for credentials. The fluorographane paper is the proof that the path was worth it.