Why MKCOL/PUT can't automatically create ancestor collections?

I understand that in RFC2518 that it clearly states that for MKCOL "..all
ancestors MUST already exist, or the method MUST fail..."

Why is this?  I can't find the rationale for not allowing the server to
automatically create ancestor collections if needed for PUT/MKCOL request
and return "201 Created".  Just wasn't sure why this is a MUST requirement
instead of a SHOULD, MAY, ...

Thanks,
Steve