Well, firstly because I roughly know, right now, what my perfect career would involve. I know the forms and shapes it could take. Not to mention the fact that I have a greater chance of achieving my perfect career anyway, as opposed to finding my perfect partner.
Whereas I have no idea as to who my perfect partner is (what traits they have etc...). So suppose that this omniscient being, who proposed such a dilemma, could tap into my subconscious and draw out something that I have no clue about and create my perfect partner. I'd be compelled to pick the partner just because it holds an element of fantasy. I.e. I will probably never meet this person in reality. The difference between an average partner and a perfect partner would probably be larger than the difference between an average job and my perfect job. In the latter respect, it would be more difficult for me to judge where my average job finished and my perfect job started.
Another criterion you said is that you must sacrifice one for the other? Fine with that. I'd rather have no job than no partner. So even if I couldn't ever do medicine (another thing: since I'm not halfway through being a doctor, I wouldn't
really know what I'm missing in the first place), I'd be content doing other things/hobbies. Like cooking. Or reading. Or writing. Or playing golf. So having no job is fine. Obviously money is still important but, hopefully, my perfect partner would be rich enough to provide for us both
Whereas the idea of having no partner would, definitely, be a bad thing IMO. I'm not saying I'll be emotionless/a loner if I don't have a partner; I just feel that I need that connection.
Meh. People want different things.