Why just brake lights? Headlights don't exactly shine and reflect and glare like they should either. Truly realistic, dynamic, real-time lighting is very difficult and resource-intensive. I'm not sure anybody even can do it right yet, let alone make it work in a proper gaming situation without making huge sacrifices elsewhere.
They could probably make things look a little better than they do, but none of us know exactly what can and can't be done (or how easy something might be) in the game/lighting system they use. It's not as if you can't see brake lights light up at all at night, though they probably should be a bit brighter. But then people would probably complain that it was distracting.
If they were to improve something, I'd prioritize making all the cars' headlights actually shine on the track or making it more bright in your cockpit/mirror when someone's bright headlights get close to you rather than making brake lights prettier. I just love having incidents because you had no idea a car had closed in behind you at night.
