@applejack
Because that is not, what is being done. When you showed me the example, I also did make 10s in mmy head. Obviously, that is the superior method.
So, you can clearly understand, that the understanding of the making 10s was not locked behind the common core.
The point actually seems to be to delay the abstraction of numbers from objects into digits. You do know, that the number 13 represents ............. but it's easier, faster and less prone to mistakes, when you can write 2 symbols instead of 17. This also delays the point, where kids actually start thinking about the abstract systeems instead of thinkking about this long and complicated process.
YES, I SAID IT. The common core is more systematic and less abstract, then the old way. Therefore, it will lead to lesser understanding of why abstraction is useful.