Why do so many people have such difficulty with comprehension?

The OP has posted the guidelines and then stated their personal opinion which differs from the quoted text. The operative words here are "that can be delivered to your letterbox", but unfortunately, far too many stop reading as soon as they see the word "Letters".

With some letterboxes having a narrower opening than the size of a regular letter, mail must be folded to fit in the slot. As an example, think of the hooded cast letterboxes mounted in brick/concrete walls which are extremely common in older houses and unit blocks.

Staff other than the SPDO have no knowledge of the letterbox size at any address (other than their own) - and in many cases have absolutely no idea where the recipient lives - so they must assume that all letterboxes are of the smallest dimensions.

If a simple "Please Do Not Bend" sticker is added to a letter, then it in not able to be delivered to such a letterbox, so it automatically qualifies for Parcel Locker delivery.

It's not rocket surgery.