If someone comes in with solid C++ abilities and not too much else, it's assumed they'll pick up other things as needed. Whether that's justified or not, that is an intrinsic widespread attitude in the industry. It's never assumed that a junior hire will just pick up C++, so the company basically plans to have to train you for a while before you're capable. And if I'm choosing between the candidate I have to train or the candidate who is ready to go out of the gate, well... it can be overcome, but it's easier just to have that knowledge up front.
This comment is ONLY applicable to the game industry. There are many types of software development that don't have this expectation.
I'd disagree to some extent. If you have a strong expertise in a particular field that a games studio requires, i.e. maths/physics and you have coding experience but not C++ then I think said studio will assume you can pick up C++ on the job. I know a lot of seniors I've worked with in the past started out like this.