Because something's wrong with them. It's either in the switch, the power supply, or the light assemblies themselves. Since they're both out, I'd look more at the switch, especially if your turn signals and four ways are still operational. You'll need to diagnose it... remove the brake light switch, check to see if there's continuity when it's depressed. If there is, you'll need to see if there's power going to the switch... if there is, then you're likely looking at a wiring fault somewhere between where it the brake light switch and the point where it splices into that circuit along with the turn signal wiring.
Check the brake light switch.
The brake light switch is broken or out of adjustment. The brake light switch is attached to the brake pedal lever above the brake pedal pad.
The brake light switch is either broken or out of adjustment. The switch is located above the brake pedal.
fuse, bad turn siginal switch, bad bulbs, bad bulb socket, bad brake light switch mounted under the dash at the brake peddle.
put the parking brake on the first click. it wont engage the brake but will turn of the lights but if you have a stock radio it will start beeping at you as soon as you hit about 10mph
They are burned out.
You need to CHECK the brake light FUSE with a test light. If the fuse is okay then check the brake light switch. That is just above the steering column under the dash. It is H..L to get to.
Those tail lights could be the brake lights you are seeing and that would be a faulty brake light switch or the brake light switch is out of adjustment.
becaus the 2004 Chevy impala does not have automatic lights and they have to be turned off manually
Check the brake light switch down on the brake pedal.
Check the brake light switch adjustment.
Brake light switch is broken or needs adjustment.