Why don't freezers have lights when fridge compartments do?

This comes down to the Cost-Benefit Principle, which states: an action should be take if and only if the extra benefit outweighs the extra cost.

Producers have no incentive to add an extra feature to a product unless it adds sufficent value to a product to allow them charge enough to more than cover its cost and complexity.

This is the technically feasible versus commercially feasible evaluation companies must under-go. Just because we CAN do it doesn't mean we should.

Axiom:
The perceived value of having a light in the freezer compartment of your refridgerator does not justify the added cost and complexity.

Example:
The light in a fridge compartment is useful only when their is no additional lighting. This is typically late at night when you stroll into the kitchen for a midnight snack. Given this "primary usage scenario" it would be very unlikely that individuals would be "midnight snacking" on ice lollys, ice cream and frozen peas. Therefore they would require lighting only for the fridge compartment and not the freezer compartment.

Possible Exceptions:
On a rare occasion they may require a light in the freezer, but these instances are so few and far between users would not typically have sufficent usage to justify the additional cost. For those that do have that requirement there exists such high-end refridgerators with a starting price at around £10,000.