Why don't companies who ask these kinds of CS questions in interviews just put in the job ad something along the lines of "if you pass the initial filter we'll invite you for an interview. We'll grill you on some data structures & algorithms & give you some whiteboard challenges".

Which is absolutely OK. If that's what you're going to do - regardless whether it's sensible or not - then potential candidates can either ignore your ad, and save everyone's time, or apply to you knowing what to expect. If a large enough number of desirable companies do this a candidate might decide to brush up on CS 101 and interview practice.

That's honest and courteous, instead of ambushing candidates in the interview stage - when they've taken a precious vacation day and taken the time flying out to see you. And it saves you time - any candidate you get should know what to expect.