No dissertations I guess.
But you make a good point. It's hard to believe that Spock never got a PhD. If anyone could do it, devote the time to finishing a dissertation and be a Starfleet officer, it's Spock.
Same with Data. He doesn't even need to sleep.
Probably Dax as well.
Or Scotty. My gosh, he even figured out how to transport someone to a starship in warp!
Now, having said that, I have a PhD in AE and I've worked in the space sector for years, primarily in mission design although I've done some propulsion modeling as well. From my experience, there are so many incredibly smart guys and girls with Masters degrees. I've worked at an FFRC which wanted most of its employees to have graduate degrees and most people did have Masters degrees. They are extremely capable, creative, and smart folks who are great at their jobs. Honestly, imo, you don't need a PhD to be a great engineer, at least. If you're determined to work in academia, or at a kind of halfway house of an FFRC or research lab, then fine, get a PhD. However, it's been shown in studies that you won't make as much money over your career if you spent your time in grad school as a TA/RA/Postdoc. I think that might be all there is to it. All these characters didn't want to put in the time to do a dissertation, which can amount to indentured servitude. It's a hard, slow grind, and you might get nothing and have to start over if you advisor didn't give you a good suggestion for a research topic. I know at least 4 people that that happened to and like 3 of them dropped out of the PhD program. You have to pass an oral and written, cumulative exam and a final defense as well. And you're living on a stipend that, where I went to school, barely kept me afloat.
So, again, there are probably good reasons why some of these characters at least didn't get PhDs. Geordi Laforge said something to Dr. Brahms in one TNG episode that he's not really one for research or writing papers. See, that's the kind of guy I'm talking about, someone who has a Masters degree and is obviously a great engineer. Same with Mr. Scott!