Because it's classless. You inveigle a candidate to spend their time on a test you owe them the courtesy of feedback.

I once spent two hours on a test for a contract development company. I'm an accomplished senior developer and normally i would have told them to F off, but I was curious so i did it. I did pretty well on it, but never got a response. Zip, nada, nothing. I emailed them multiple times and even called HR. Still no response.

Anytime other devs ask me about that company, i say don't waste your time, they are super shitty.