Question

Why is an object's density expressed as a combination of two units

Answer 1

Well, first of all, EVERY physical quantity is measured in a combination
of 2 or more units, except for mass, length, time, and electric charge. 
ALL other units are made out of these.  So it should not surprise you.

[  Example:  Speed  =  (length) / (time)  ]

Density is not the mass of a substance.  It's the mass of a substance in
a standard volume of it.  So the density is made of the mass in any lump
and the volume of that lump.  That way, no matter how much of a substance
you have, you can always compare the lump you have to all other substances.