>Does one really ask a Mechanical Engineer or a Civil Engineer about their side projects?

Yes, sometimes. I'm a mechanical engineer, and frequently interviewers would ask what sorts of projects or engineering related hobbies I have. They're not necessarily looking for me to have brought a mass-produced consumer product to market in my spare time... most mechanical engineers I've met seem to be excited enough if you talk about working on cars or bikes.

I can see how software engineering may be similar. They may not be expecting you to be working on some hugely popular app, they may just be trying to get a gauge on what you're really passionate about. I don't like to wrench on cars in my spare time, but I've still gotten job offers after it comes up. In the end it's just a matter of finding the team that has similar values to you.