Why don't rally cars have airbags?
Other competition cars, including Ken Block's don't have airbags either.
Is it just for weight reduction, are they redundant or is there another (major) reason?
For the most part, they don't need it. The purpose of the air bag is to protect the occupant as the body is thrown forward in a crash. Most racers have such vastly improved safety equipment over what is available in a standard road worthy automobile, they don't need air bags.
The things you'd need to take into account are things like a five-point harness, which pretty much holds the driver in the seat and does not allow him to shift forward, even in a crash. A standard automobile has a 3-point harness, which keeps the body in place for the most part, but is no where as efficient as a 5-point is.
Most racers also use what's called a HANS Device. This keeps the head in place during a crash, alleviating any strain on the neck during a high speed incident.
These two pieces of equipment make airbags pretty much useless in a race car. With these safety items in place, the body doesn't go forward, but rather is kept in the seat and is better protected.
If you are ever lucky enough to drive a competition car, the first thing that will strike you is that you can't move. Once you're in your bucket seat with your fire retardant overalls on, wearing a full face helmet and neck brace and strapped firmly in with your multipoint harness, you'll feel like you've been pinned to the seat.
You can't look over your shoulder like you can in a road car. You can't reach the location where the glove-box is. In the event of a collision, the chances of your helmet coming into contact with the steering wheel are absolutely minimal.
However, if you watch rally cars in action, especially on gravel, you'll see them yumping (rally speak of hitting a jump), you'll see them in and out of ditches, ruts and rivers. Many of the impacts a rally car suffers during normal use would far exceed the trigger points to deploy a standard air-bag. Deployment of such a device when landing an airborne car that's travelling in excess of 120mph (200km/h) with spectator lined roads could have deadly consequences.
Similarly, in racing when there are multiple cars jostling for position on the same piece of track, deployment would be likely from all the "door handling" (a term to describe "acceptable" contact between race cars) and if one in the middle of a pack were to deploy, forcing the drivers hands away from the wheel, it would likely cause carnage.
So, in summary, air bags are not fitted to competition cars because they could pose a serious and significant risk to both the occupants of the cars and people outside such as other competitors, marshals and spectators.
Simplicity (less to break, repair)
Safety: There would be more instances where they might cause more harm than good. (for example minor impacts or large jolts might trigger airbags, thereby rendering the car and driver incapacitated... when they would normally still be able to drive)
Safety #2: The drivers have already are wearing advanced harnesses and helmets which provide a similar restraint effect as airbag.
Summary: They wouldn't really serve a purpose other than getting in the way
As others have said, there are two reasons - one is that in a rally car, you're very firmly strapped in with full harness belts, as opposed to the inertia-reel belts in a normal car. The other is that a rally car will have many bumps and jolts that would set off the airbags in a normal car.
I'm a regular rally navigator (or co-driver), it's actually quite impressive just how little you move around when properly strapped in - which means you can concentrate on keeping your place on the maps/notes over the rough bits instead of having to hang on! It also means the driver has a lot more control, as again they are not moving in relation to the car, so they can put in much finer steering input.
It is pretty straightforward, it has absolutely nothing to do with simplicity weight or anything like that! You are not allowed by fia rules to have working airbags in a competition car due to the fact that during racetime any bump or contact to the impact sensor could trigger the airbag, meaning you get punched in the face by it and have no visibility temporary due to it inflating. Now imagine you are going 100+ kph with a corner coming and people on the sides or a giant drop. It wouldn't be a very fun situation. Furthermore, when you are strapped in with a harness, it renders the airbag completely useless as you don't come into contact with it
All answers are good but one simple thing missing. Airbag is sensitive and on small crash can be opened. If you race and made small crash you can continue racing if all most important parts of car works. Also if car start to fire, airbag will keep you inside and you are dead. Any help for you or any kind of fast excape with airbag is hard.