I'm sure it's true that a non-trivial number of good candidates will reject a job opportunity when asked to do a work sample. These, however, will generally be those with plenty of attractive alternative offers - the ones well known in the community, with a good network etc.

At the other end of the stick, there are those still trying to break in to the field. They may have moved to a new city, they may have worked in a totally unknown firm, they may have taken an "alternative" path through the educational system (and just to be blatantly obvious, the above describes me, circa 5 years ago) - their CVs don't jump out at employers, and as even phone screens are fairly expensive in engineer time, they are not very likely to be given a chance.

Online coding challenges and take-home tests etc are a quick and cheap way to extend an opportunity to more "risky" candidates. I think that's an important element.

(FTR, the company I ended up with, I got through to primarily via a "Who's hiring" thread on HN, however they did operate a "take home" test that took one hour).