The role of the combustion chamber is to burn as much of the fuel+oxidizer as possible; never discard any unburnt fuel or oxidizer because it had to be carried there at huge expense of fuel and oxidizer earlier - every gram counts.
Jets carry only fuel, they have air available in abundance, so as long as all fuel is burnt, surplus of air not having reacted with the fuel doesn't hurt - and actually helps; heated it expands and provides thrust, without need for huge exhaust velocity which would be hard on the turbines; more gas expanding by less, instead of a small amount of gas expanding by a huge factor - in rockets a tiny amount of mass provides a lot of thrust. In jets, the amount of mass carried by the plane is even smaller, but the mass providing thrust - intake air - is much larger, the airplane over a single flight pushing many times its own weight in air through the engines.
And then there's aerodynamics. Refer to this question.
The two engines on the left have nozzles for atmospheric use. By the time the exhaust gas reaches the opening, its pressure isn't much higher than atmospheric, and it can't provide much more thrust.
And this is the bell nozzle attached to the third of the above engines - meant for vacuum.
Every last bit of momentum is squeezed from the exhaust gas, which would otherwise escape uselessly sideways.
Installing such a thing on an airplane would be completely counter-productive because the huge nozzle itself would introduce so much air drag (through its outside in the air stream) it would completely nullify all the benefits.
Although, your question does have a significant merit. Nothing beats the bell nozzle in void; it's the most efficient way to harvest momentum out of gas expanding into void. But bell nozzles for atmospheric engines are a crutch, an unoptimal reduction of the void bell nozzle problem to border conditions of the atmospheric pressure. They work, they work well, but they don't work optimally.
The counterpart of the nozzle of an airplane jet engine in rocketry is the Aerospike engine.
Video of aerospike in action.
The aerospike is definitely superior to bell nozzle in atmospheric conditions. The problem though, is that large-scale implementation of aerospike engines would require a lot of new research, while bell nozzles are 'tried and true', tested, well known and readily available. And so, because nobody wants to pay for "being the first", we're still stuck with bell nozzles for atmospheric rocketry.