Because the the bigger point behind the grand staff for piano is to have one for each hand rather than show an absolute not position on the instrument. It's not too uncommon to see two treble or two bass clefts in the grand staff. For example the way Imagine by John Lennon is notated for piano is using two bass clefts.

There are also other modifiers that can make the top or bottom staff different like octave clefs, ottava alta marks, and ottava bassa marks. It's useful to teach the grand staff the way you mentioned, but in practice when playing keeping them their own systems is important.

Because the the bigger point behind the grand staff for piano is to have one for each hand rather than show an absolute note position on the instrument. 

It's not too uncommon to see two treble or two bass clefs in the grand staff. For example the way Imagine by John Lennon is notated for piano using two bass clefs.

There are also other modifiers that can make the top or bottom staff different like octave clefs, ottava alta marks, and ottava bassa marks. 

It's useful to teach the grand staff the way you mentioned, but in practice when playing keeping them their own systems is important.