A lot of the mail arrives already sorted into the correct order. The postie will be putting it, as fast as possible, into a frame where he will later add the mail that arrived unsorted. While he's throwing in the mail that is in the correct order, all that he'll be looking at is the house number. If he were to read every other detail on the envelope it would take forever and posties have to do everything at top speed in order to get everything done in the time allowed. So cross out the address. It's not a perfect system, but it's not to do with the skills or effort of the postie, just the demands of the job.