That is a very good point. Seriously. I mean it.
But unfortunately there is a small problem (that potentially could be solved somehow).
Any company at which we pay for our certificates would listen to the paying clients and go out to hire a good technical writer. The newly hired resource would have to learn the innings and the outings of how the 'client' software interacts with the service provided by the LE server run by the LE team. This newly hired resource would then have as a task to write down, and constantly maintain, this information in a language 'anyone' could understand. That's a huge task, though many companies do this, with more or less success.
The company will have to pay a salary to the tech writer. Taxes. Social security. Deal with sick leaves. Vacation. Ordinary stuff like that. It doesn't come for free. It's expensive. No problem. the company gains profit from the sales of certificates.
Let's Encrypt is run by enthusiasts, on their free time. If anyone of the LE people get any kind of economical reward for their time and efforts, I'd be surprised, because I fail to see where the required revenue would be generated.
So, "things must be better" is probably a true statement in most software projects, including LE.
How to make it happen? Well, someone must step up and say something like: "I'd be happy to spend hundreds of hours learning the details of how this thing work. I'm also very skilled in writing very user-friendly documentation, including pictures and use cases. I'll be happy to help. Where do I sign up?"
Now, when that is all said, it's quite common to see people studying the shoes they wear. Or the more straight people saying things like: "But I'm busy. I've got a daytime work. And a family. It's summer and vacation-time. I'm sure you understand that I can't find the time to do all that."
Well, all those time constraints are valid for open source developers too. Still, they sometimes ignore their families, their vacation and sometimes even their day-time work to give you access to free (as in beer) TLS certificates that you can use on the Web server computer that you do administrate.
I'm also aware that things "could be better". I agree with you on a number of points, though not all. But the right approach would be to humbly point out what is unclear (in detail, with references, making the life as easy as possible for the developer team) and humbly ask if it could be approved upon. From there, proposing your time to help... Well...