Why don't bicycles have single lever for both brakes?

It has probably to do with the mechanical inaccuracies of the system.
The available angular movement of the brake lever is fixed. If one or other brake has worn out more than the other and one lever operates both, then that brake will hold less or not at all.

Even if one of the brakes gets worn out, the other should continue to function as an independent brake like it does in current bicycles. I'd like to know whether using both brakes simultaneously is a good idea or not. If yes, why? If not, what's the reason?

The theory is that If only one brake is used it has to be the rear brake. If the front brake alone is applied, there can be a hazard due to the inertia of the vehicle acting on the immobilized point of contact of the front wheel with the ground. Under certain conditions this can lead to toppling.
Since the brake action is related to the force applied by the hand pressing the lever it will certainly be better to have both brakes acting simultaneously.

Kaustubh KatdareI'd like to know whether using both brakes simultaneously is a good idea or not. If yes, why? If not, what's the reason?

Applying both breaks simultaneously is best for BIKES, it give sturdiness and quick stop.
In Bikes, rear wheel pushes the front wheel while acceleration. So I think there should be different break pressure while applying break. Mostly both breaks simultaneous with front one little gentle and rear one with harder press give very good result for stopping Bikes.
But if there is only one break for both wheels then, how it should be calculated that how much breaks should be applied on each wheels individually.

PS:I am taking Bikes means both motorcycle and bicycle.

Using individual breaks for both rear and front wheels makes sense in motorcycles. What about bicycles?

If you have single lever to control, what if it fails?

Yes all are correct in some point of view.I really agree to Ramani sir answer about the moment.As you are saying that a common lever for both front and rear brakes, If the lever is cut out it can lead to accident(because we will be not able to apply both front and rear brakes) added that if we ride the bicycle at high speeds and if brake os applied through single lever , It may skid forward.

Heard of the "combi-brake system" in the Honda Activa ? well in this, one of the brake levers is connected to both the front and the rear wheels.

well i know that i am discussing a motor-scooter in a thread meant for bicycles, but they both count as two-wheelers, don't they ?

but they both count as two-wheelers, don't they ?

Yes, but as chalk and cheese. The precision of machining and assembly in the two are quite different.

zaveriHeard of the "combi-brake system" in the Honda Activa ? well in this, one of the brake levers is connected to both the front and the rear wheels.

well i know that i am discussing a motor-scooter in a thread meant for bicycles, but they both count as two-wheelers, don't they ?

Yes today hero motor corporation had said that it will be introducing new technologies which were used in cars.One of the system is know as "IBS"-Integrated braking system where both front and rear wheel brakes were connected to a common lever.Along with this Hero also ready yo introduce Stop start system and electronic immobilizer.

SarathKumar ChandrasekaranYes today hero motor corporation had said that it will be introducing new technologies which were used in cars.One of the system is know as "IBS"-Integrated braking system where both front and rear wheel brakes were connected to a common lever.Along with this Hero also ready yo introduce Stop start system and electronic immobilizer.

what is this electronic immobilizer ?

zaveriwhat is this electronic immobilizer ?

Electronic immobilizer is vastly used in four wheelers but presently not available to two wheelers.Hero Motor Corps would be the first to introduce it in two wheelers.

Electronic immobilizer- is a anti theft device which helps in securing vehicle from hot - wired(by passing the ignition circuit).
In this type of device, key will contain a small circuit which should activate the ignition process.It will be controlled by Electronic Control Unit.
refer following link to get a clear idea: en.m.wikipedia.org/wiki/Immobiliser

There are different usage for front and rear brakes in any vehicle (including Bicycle). just take an easy example -

You are going down from a hill, applying front brakes will have you topple over the handle, similarly going up the hill will have the topple effect when you will apply rear brakes.

Sliding, skid, topple are the few things that depends on the front and rear braking.

So regardless that mechanically it is possible to have one lever or not, i don't see much of the benefit of this. Correct me, if this doesn't make sense?

-CB

Anil JainYou are going down from a hill, applying front brakes will have you topple over the handle, similarly going up the hill will have the topple effect when you will apply rear brakes.

Not really. Front brake leads to the toppling tendency whether one is going up or down hill or on flat ground. Please check the inertia issues and you will find out why.

here is latest, not cycle but new Honda CB trigger have combi breaking system.
If you apply rear break front break will automatically applied to give you smooth and better breaking.