My first question was asking what a field biologist would do in SF.

No, your first question was:

Why would a field biologist live in a city?

This question doesn't really make sense, unless you assume that living in the city precludes them from doing their job in some way. It assumes that there is no FB work in a city, as well as assuming that you cannot commute out of the city to do FB work. There are very few jobs like that. Someone could say that they are a lion tamer and live in SF and I would either assume that there is a place to tame lions here, they commute to one, or they travel for some period regularly to do so. An astronaut could live here there's no reason to say "Why do you live in a city?"

As for your 2nd question, whether or not you used IF with the statement " those jobs do not exist in San Francisco" doesn't matter. Those jobs exist in or around SF, or they exist in a way that allows her to travel to them.

But none of this matters. What matters is that you understand that even if someone has a job that makes their commute seem unacceptable to you, it may be acceptable to them. Where you live is very important to people. And not just City Vs Suburbs Vs Rural, but even the specific place. Even if we go back and assume there isn't a job nearby for this woman, your comment is basically "Why would someone live in a city if they can't do what they were trained to do there?" And the answer is either "Because this is where she wants to live." or "Because this is where she lives and she can't figure out how to leave." We usually don't assume the second one right off the bat.

So your question is right up there with "Why would someone pay so much to live in SF if they presumably could live somewhere less expensive?" And the answer to that (without intending to be pompous about SF or disrespecting any other place to live), encompassing all the different possible reasons why, is simply: Because that person wants to live here.