It sounds to me like a way to weed out pesky applicants who have families or who are simply older.
People usually trust their own assessment of a candidate much more than that of others. While your projects might help generate interest in you and get you an interview, the actual interview process is meant to be an assessment by the Company conducting the interview. So you shouldn't automatically assume you have the jobs simply based on your past projects alone. I'm not saying I necessarily agree with how this works; I am simply pointing out why it works that way.
> It sounds to me like a way to weed out pesky applicants who have families or who are simply older.
Perhaps. It seems unlikely since many of the senior developers/hiring managers at most mature companies are older and have families.
That's insane. If you brought me in because you liked what's on my resume, your number one priority should be determining if I really did what I said I did. Your priority shouldn't be arbitrary, unrelated questions or coding challenges pulled off some website.
I'm not aware of any other industry that behaves like this.
There generally is a component where you're asked about your past projects in detail. Its just not the only component.
I kinda agree with you that it doesn't make sense much of the time if you have to specifically prepare for the coding interview; stuff you may never use in your job. But its not a lot of stuff: I bit the bullet and spent some time solving those questions and now can make past mostly any screen.
Its really not that hard, especially if you have a CS degree. Probably would take 1 week of dedicated effort to get better at it.