[...] Obviously any number past 9 has to be sent in Morse as separate digits, but why would anyone say a number in a sequence as anything but "forty-six" or "seventy-three"?[...]
To keep the risk of error or misunderstanding at a minimum.
For example, aircraft controllers and pilots ALWAYS say each digit separately for aircraft id, altitude, altimeter setting, squawk number, etc. Its always runway three six, never thirty-six.
Obviously, for a non-critical communication code like 73, there is no danger to life or safety, regardless of the manner in which it is said.