That could be because putting it on the box creates a liability that the LEDs are meeting those specifications -- just because the source LEDs claim to meet those specifications doesn't mean that the enclosure being sold using them as a component will actually produce that. Additionally, any change to LED suppliers/etc now means that the box also has to be changed.

In other words, what they are selling isn't the same as the thing in the box nor the aggregate of all the components (since they interact with one another).