I’ve been doing some research, and there are three main reasons:
1. Because the temperature in a freezer is much lower than of a refrigerator, a much more expensive shatterproof bulb would have to be used. In fact, if you put a common household bulb in the fridge, it will be fine. But put it in the freezer, and it should crack after a few hours (or days, depending on the temperature you freezer operates at).
2. Because there is less demand for illuminated freezers and yet midnight snacking almost exclusively demands a refrigerator lamp, lamps are often only fitted on the higher end models.
3. Standing freezers often use closed-off pull out drawers. In order to effectively illuminate the entire fridge, a lamp would be required behind each shelf. Each of these would need to be shatterproof… you get the idea.
Hope this helps.