Less flippantly: pedagogically, there's a big difference between "here's your addition tables, memorize them" and "if I have 1 of _anything_ and another 1 of that same thing, I now have 2 of that thing." The latter offers way more opportunities for further thought: by that logic, if I have 2 things and I take 1 away, I now have 1 thing! If I have 2 rocks and 2 sheep, I can count the sheep by laying out 1 rock per sheep! And since I can add more things, maybe there are more numbers for those amounts of things too? And what about differently-sized things? Or parts of things? Or...
That's the difference between getting "1 + 1 = 2" across as a literal by-the-book fact, and getting it across as an invitation to build / connect ideas and ask further questions.
I'm pretty sure that is already how addition is taught. I'm not a fan of formal education at all, but I'm not sure what you think schools are missing here?
"seem perfectly capable at getting at least that across" yeah, that's part of the problem. They already know that 1+1=2, or so you'd think. But you would be hard pressed to find a teacher who can actually explain WHY 1+1=2 (which is based in how our number system was created and using proof by induction). Anyway, that is besides the point, since you won't teach children math with college approaches, but the important thing is that most teachers don't understand what they teach, they just "say what they remember".
And that doesn't pair well with children's insatiable "why" requests, even if they don't utter them. Teaching children is actually harder than teaching adults, because most adults largely gave up on the "why" and just settle on "I gotta remember that, if I want to pass the next exam" (school did a good job with them I guess).
So why 1+1=2? There is a lot of depth in that that is left unanswered and children are forced to memorize the answer. From then on, a sharp decline of cognitive ability follows as they "graduate" through our excellent school systems...
Poh-Shen Loh has a math course for middle school kids, and a weekly live interactive youtube stream specifically to answer questions kids ask teachers.
One on his channel is to explain to kids how the determinant works, why matrix multiplication is done in that specific order, etc. Check them out sometime esp his Friday 'ask math anything' live lectures.
Less flippantly: pedagogically, there's a big difference between "here's your addition tables, memorize them" and "if I have 1 of _anything_ and another 1 of that same thing, I now have 2 of that thing." The latter offers way more opportunities for further thought: by that logic, if I have 2 things and I take 1 away, I now have 1 thing! If I have 2 rocks and 2 sheep, I can count the sheep by laying out 1 rock per sheep! And since I can add more things, maybe there are more numbers for those amounts of things too? And what about differently-sized things? Or parts of things? Or...
That's the difference between getting "1 + 1 = 2" across as a literal by-the-book fact, and getting it across as an invitation to build / connect ideas and ask further questions.