Why do people say only god knows?

As an agnostic. I gave a bunch of religions a try. Whenever I asked a question they couldn't answer they say only god knows. When it came to understanding the planets before rockets, people always said it was gods work when explaining how the planets move etc. Then we have a understanding why some planets do what they do and they no longer say its god. Why cant religious people say they dont know something instead of using god as an excuse?