I think, the "source control question" comes from the Joel Test which is 19 years old. Almost every company uses source control now so it doesn't make as much sense.
A better question would be "why do you use [the source control they use]". The answer, if coming from a knowing person, may tell you quite a lot about how the technical decisions are taken in the company.
There was definitely a time when that was an important question, because the answer could be "CVS", "nothing" or even "what is that?". Nowadays the answer is practically always "git".
Indeed. But why git? I worked for 3 companies that adopted git for completely different reasons.
The first was open sourcing parts of their code so they just started using GitHub. Makes sense.
The second used to work with Subversion, and they stayed with it. The process was built around having one and only one server with all the CI, and the hooks, and the locks. Git was used with the SVN gate purely for developers' convenience. Makes sense.
The third one was on TFS, and they couldn't quite align their process with it. So they thought "our process is a mess so let's do git". This is a red flag. If you can't get your process straight with relatively simple tool, some more sophisticated tool will (and it did) only make it worse.
Honestly? Because it's become accepted as the industry standard. People who need to make the decision without having knowledge about the topic just go with whatever everybody else is doing. In the past, that used to be whatever IBM was offering, then whatever Microsoft was offering. In that respect, git is a lot better. It's flexible enough to adapt to different kinds of workflows, at least.
This is also role specific. Almost every developer is using source control. But it is not yet standard that operations and infrastructure teams are using it.
A better question would be "why do you use [the source control they use]". The answer, if coming from a knowing person, may tell you quite a lot about how the technical decisions are taken in the company.