Head 'em up, move 'em out, move 'em on, head 'em out Rawhide!

Just enough time for a deep philosophical question before I head overseas

Why are backpacks measured in litres?*

My backpack has a reasonably sizable 90ltr capacity, which would be great except for one important distinction - I'm not filling my bag with water.

This may sound like I'm splitting hairs, but think of the alternative:
1) Water would leak out of the zips, so I'd have to refill it regularly
2) It would weigh 90 kilograms**, putting my bag not only over the standard weight allowance for international flights, but be impossible to move. Well, until some of the water had leaked out.

On the plus side - I'll be well hydrated

What I am filling my bag with is assorted clothes, books, medications, shoes and other miscellany which just as easily fills up 90 litres, without the leaking.
Or more correctly, I'm trying to fill my bag. Because it seems there is always more to pack than you'd think, and while the advantage of a liquid is that it will fill all the available space perfectly, solids have a tendency to be, well, solid.

I'd better get back to it; next stop San Francisco.

* that's a metric measure of volume for anyone in USA
** also a metric measure, this time of weight