I've thought the very same thing! We have a Jayco 264BHW and always wondered why Jayco didn't provide a top-bunk ladder. Even the after market fixed-length ladders that are tall enough, are always in the way of something like the bathroom door, and/or stick out too far into the walking area.
Fortunately at almost 8 years old, our grandson is like a little monkey and hasn't shown any need for a ladder. Most of our camping trips we just use the top bunk for storing extra grocery supplies.