There’s a cynical voice in my head that says “because no-one has ever asked them to”.
Where I work we spend a long time looking before we hire someone, because it’s so hard to find them. We use a walk-through test that has a sequence of “questions” leading to a simple app and are willing to argue the toss (I was hired despite arguing that the test had significant flaws: “why would I do that? That’s stupid because…” when they wanted me to code a solution that led to the next question).
I fear that any take-home or pre-interview component would lead to plagarism or “helped” solutions, so we make candidates perform in front of us.
That’s got our ratio up to 3/4 good hires.