That seems like a really serious flaw. Did the teaching community at least make and share their own?
The typical cycle for curricula, teaching strategies, and/or discipline plans in schools:
1) OMG the kids aren't learning better change everything!
2) District, state, or both pick some new system and pay some consultants and various companies a bunch of money.
3) Administrator presents plan to teachers: Here's the new system. Half of it's not finalized yet. Should be done in a year or two. We're not implementing some parts of it because they make me uncomfortable. The old way is stupid and sticking to even the parts of it that work well for you will result in my making your life miserable and eventually trying to fire you. You'll need to re-write all your lesson plans to conform to the new system. Here's a mountain of new paperwork for you. No, you can't stop filling out any of the old paperwork.
4) A couple years pass
5) Goto 1
1. Introduce a new test with an unfamiliar blend of questions, new style questions, etc. but don't give teachers extra time to actually receive updated textbooks, update their classes, etc.
2. Start giving the test to students immediately before they have any experience with the new format
3. Use the result low test scores as proof that things were really bad before
4. As everyone gains experience with the new system, attribute rising scores entirely to the brilliance of the new model. Give bonuses to district / state officials.
5. Once scores plateau at roughly the previous levels after a few years, start talking about repeating the process. At no point should you seriously consider tackling the structural problems preventing holding perennially under-performing groups back.
Lesson plans from big companies like Pearson, on the other hand, are good because they come from businesses. However, there's a conflict of interest because many of the same companies write the exams. Worst of all, none of these companies actually develop teachers' abilities to teach. That's not really the fault of the companies; we just don't do that here in the US.
Teacher development is almost nil for a lot of K-12 teachers in the US, compared to Japan or Finland. The average US teacher is in the classroom 1080 hours a year and it's something like 600 in Japan and Finland. That 480-hr-per-year difference is spent on development and prep.
And yet there are a lot of really excited and entrepreneurial teachers and they're making and sharing all sorts of material. See TeachersPayTeachers, for instance.
There is also an initiative to fund CC BY licensed educational content that school districts can use without getting milked by textbook publishers http://k12oercollaborative.org/
And yes, teachers do share lesson plans, both online and within their schools.