I think it's because we're mainly focused on getting ideas right - most of the time, writing out the equation is merely a confirmation of what we allready know to be true. So often, a mathmo will write out a series of equations where the beginning will be true, the middle completely wrong, and the conclusion correct.

I wonder why that doesn't work in cryptography. There are several well-known examples of "security proofs" (proof of security of a crypto scheme under the assumption that some computational problem is hard) by respected researchers that turn out many years after publication to contain errors that render the conclusions invalid.

Or does this happen just as often in mathematics, except that mathematicians don't care so much because their errors don't usually have much real-world impact?