Then you don't really understand what the job of a mother is, or more specifically what it could be. It's like suggesting that people are only effective at work if their boss is standing over their shoulders.
My wife has an advanced degree and she's being much more productive at home (even when the kids aren't there) than she would be in the workplace.
Are there mothers who waste time at home? Yes. Just like there are Fathers who waste time at work.