The literal meaning seems to be :
How could it not be a driver not a baby ?
I presume it has another meaning.
Any ideas ?
It all comes to how Russian names (and maybe names in Slavic languages in general?) often end with 斯基(-sky) for males and 娃(-va) for females in their Chinese translations. For example:
and I'm sure you can find other similar names, either that of real-life persons or fictional characters.
And so it becomes a somewhat well-known wordplay/pun that there are only two types of people in Russia - 司机 (drivers) since it's a homophone of 斯基, both pronounce as sījī (si1 ji1), and 小孩 (kids) since it's basically what 娃 means in Chinese. And when you put it that way, it's kind of funny, so people remember this joke.
Now back to the original question, 怎么不是司机不是娃呢, it means: How could someone neither be a -sky nor a -va? But of course, if translated this way, the humor in it is lost.
By the way, I imagine playing jokes with other people's name can be disrespectful in the eye of some people/culture. However, it's unlikely the speaker had any bad intention. If you find this rude or if it makes you uncomfortable, you should respectfully tell the speaker to stop making this kind of joke, and tell them how you feel. We can all be careless sometimes when interacting with a person from a different cultural background. I'm sure any respectable person will be more careful upon your request (or, at the very least, you will see the kind of person they are).
If you are not a Russian or you don't mind though, feel free to ignore this section. 😄