Why make cheese if milk is already food?
Solution 1:
Milk rots. I'm fairly sure cheese doesn't - or at least does so at a much slower rate.
ETA: Apparently, it does rot, but cheese is ten times as valuable as the milk it was made from - and turning milk into cheese frees up the jug you were using.
... And, of course, is an extra task, meaning extra job experience.
Solution 2:
I think cheese is worth more, as it can have quality.