3 most important motivations are: 1) Love for science (it is hard to do full-time funded research without a PhD) 2) Prestige 3) Desire to work for the govern

3 most important motivations are: 1) Love for science (it is hard to do full-time funded research without a PhD) 2) Prestige 3) Desire to work for the government (some high-profile public service positions make it easier to get in with a PhD). Nearly nobody is doing a PhD for "employability" (except specific target positions from #3) or financial reward.