Okay, so it sounds like what you're saying is that it's risky for a familiar, not that they actually fail every single time.
A spider has a +4 stealth and a weasel has +5. So they're definitely going to make at least some sneak checks. They sneak up to the room the dire wolf or whatever is in, the wizard becomes aware of it as a result, and the familiar withdraws. There's no automatic detection taking place. Furthermore, I can't really see a dire wolf or sabre tooth tiger caring, even if it does notice a tiny spider crawling on the ceiling.