isn't kage bushin a "forbidden technique"?
thats probably why nearly no one use that technique, not only is it chakra inefficient (it was mentioned in the anime like 20 times) but it was also in the scroll with "forbidden techniques" naruto stole in the first episodes.
and it seems like not everyone knows about this side effect, kakashi said so because he too can make kage-bushin and thats why he knows about it.