Why would an interviewer assume a front-end dev has a CS degree or has invested personal time in studying algorithms rather than other (probably more) relevant topics, such as various frameworks?
That depends. Mostly because front end job listings still often list a CS degree as a requirement. Also, I think it's great when people enjoy working on the front end since I don't particularly, but I still want to work with people that aren't hyper specialized.
I think you might be underestimating the breadth of front-end skills required to properly develop, test, deploy, deliver, monitor, analyze, and optimize front-end applications. Including the myriad third-party integrations that other teams need, from CS to marketing.
Well, I wouldn't have considered half of those things as part of "front end development" work, but it pretty much drives my point. I'm a firm believer in the idea that you'll be more effective at all of those things if you have a general baseline knowledge of CS, so that if you need to you can (relatively quickly) understand how and why all of your tools work the way they do.