There's that, but if a site has forms that are difficult or confusing to a portion of their customer base, they might see a significant number of exits during other portions of the transaction as well... To maximize the number of completed transactions, they have to tune their checkout process to be easy for not just tech-savvy forum goers like us, but also for people like my mother who couldn't compute their way out of a paper bag even if provided with detailed instructions written using only small words.