Are we teaching or telling?

Why don't instrumental teachers teach their pupils what a clef is for (and their proper names eg G Clef & F Clef) - they are not just pretty symbols; and what the numbers in time signature represent?  How can a child possibly work out a rhythm if they don't understand the time signature - my most hated phrase: "a crotchet is one beat" - only if the time signature is in crotchets!!!!  If you teach them that from the beginning then you have to 'unteach' it later on - how confusing is that?  I like the american system of half note, quarter note etc - this makes more sense of the numbers in the time signature.