The fact that publish adventures stop at level 11-13 makes a lot of sense to me. In my experience and in what seems to be many others as well, level 11-13 is the tipping point g point where the game starts to need to cater to your specific party composition, powers and playstyle, in order to provide engaging, challenging, and somewhat coherent game material.
Also, a single adventure covering levels 1-20 would either be massive, repetitive, or leveling at an even faster pace than the already fast pace of published adventures. To be done well, a single adventure would probably have to cover 13-20 specifically.
So it’s not so much that it’s difficult to write adventures for characters levels 13-20, it’s that it’s progressively harder to write for an adventure for an unknown number of characters between 3 and 6 of any of the 12 character classes and corresponding subclasses, who may or may not have access to certain key spells, and an unknown bagage of magic items.
At this point, publishers tell DMs “we showed you how to do it and your players have what it needs to make it to level 20. Here, we even made a handful of high-level monsters for you to use in a jam and to show you what we have in mind, so that you can make your villains and monsters since you know best how to challenge your group. If you’re not up to it, it’s cool, here’s another 1-13 adventure.”
Honestly, I can respect that, but I wish it was said more plainly.