MFJ Both Claiming 0; how do we always owe?

I am so depressed looking at my taxes. This is probably naïve but I would think the way it works is: if we both claim 0 then they take the right amount out based on our tax rate as a married couple so that at the end of the year we've paid 100% of the taxes we'd owe based on our earnings. Then when we fill out the paperwork with deductions and credits and claiming our 3 kids we would then get a refund. Why is this not how it works? Why don't they take enough out throughout the year?