Juniors aren't, though they are expected to show some capability in the matter (and that's why everybody demands from them a college degree).
In the old days companies were training their employees on the job. In those days it wasn't uncommon for employee to stick to a company for twenty-thirty years, so training paid off for the company. Today employee works only for couple of years before moving (or being moved) to next company, so it's not surprising nobody wants to train anybody from ground up. Training requires loyalty from both parties, which is a scarce resource nowadays.
> I'm a senior software engineer, why do I have to spend hours/weeks/months on Leet Code, Hacker Rank, etc. in order to be prepared for a "technical interview" these days?
I don't. Apparently it's a location-specific phenomenon.
> Why do technical interviews force me to re-learn something not relevant to the actual job I'm applying for? [...] I'm referring to the mathematical/academic concepts like big O, binary trees, graphs, linked list, etc.
I don't know what are you doing in your work, but in my job data structures and computational complexity are important basics. Without ability to juggle with those in my head (assessing complexity on the fly) I couldn't even start to think about anything substantial. And I'm not even fully fledged programmer, I'm in large part a sysadmin.