Probably for kind of the same reason that speed is expressed as a relationship between two units. You know, like miles per hour .
I guess the only reason is because no single unit has been invented to describe density.
The rate of doing work or using energy would always be expressed as a relationship between two units ... we would say that the rate of work is "(so many) joules per second". But the "watt" was invented, so we can say "(so many) watts" instead.
So I guess you're right. Density could be simpler to describe if we only had a unit for it. Then we wouldn't have to say "(so many) grams per cubic centimeter". We would just say "(so many) (new unit)".
Let's try it out:
"Uhhh, pardon me Professor . . . I've been working late in the lab, and I believe I've identified a new substance, hitherto unknown to the scientific community, and totally unexpected. In its pure form, the substance appears to be pink, it smells like butterscotch, and its density is approximately 27.4 Brianas. I think it's time we published these findings ... with your name as lead investigator, of course."
