That's simply because people are often in a hurry while at it, during transactions. It's good to suspend everything one is doing during transaction. That's what I do. I'm never in a haste because I know once a mistake happens it's goodbye to my funds. I do take my time to check the first four/five letters of any copied wallet address I'm sending or withdrawing to. I do same for the middle and last four/five letters too. That's if the funds involved aren't much. If I'm doing a heavy transfer, I take my time to run a complete check from the first letter to the last before hitting the send or withdrawal tab. I'm conservative like that and I haven't regretted any bit of being conservative.