So is their time. Takes candidate time to do, takes employer time to test/evaluate. Both parties are risking wasted time. You're wasting time driving to interviews and answering calls etc etc. Its the cost of doing business.
Asking someone to spend an hour on something is not a big deal, but I've seen/heard of stuff where they're talking about half a day or even an entire day. For someone with other things going on like a family (I know it may seem outlandish to younger readers here, but it does happen) and a current job, that's asking an awful lot. It does not take the company an entire day to review that exercise, making the time commitment very asymmetric.