I always thought it was weird how it's called "Next Generation" but takes place 85 years in the future.. is that how generations work? I mean, even Picard would have to be about 60 years old when TNG began to technically qualify as the generation after the original.. god I didn't even really watch Star Trek.
Probably because each light is a different brightness and they wanted to be able to adjust it in post based on how it looked. It's way cheaper/easier/faster to recombine different shots than it is to have to reshoot, especially when you're doing it on film.