It has been stated for many decades that a loudspeaker is best driven by a voltage source .This means that the lower the output impedance the Audio amplifier has the better .High output impedance gives a poor damping factor which would mean muddled bass response due to poor time domain performance.If an audio amp was driving an 8 ohm speaker that had a DC resistance of say 6 ohm had a total output impedance of less than say 600 milliohm then the sound would be tight .Solid state Audio amplifiers achieve this with ease by Emitter Follower output stages and lots of negative feedback .Some valve amps will have problems achieving this .Headphone outputs on stereo amplifiers use a series resister in each channel to limit the power to avoid accidental headphone or ear damage .A typical resister would be 220 ohm 2 watts .Most stereo headphones are 8 ohm nominal impedance and their internals resemble a small dynamic loudspeaker .The damping factor of such a scheme would be very bad .Why was this done ? Do headphones not mind a poor damping factor ? Was this just cost ?
Cost savings. The headphone jack is usually an afterthought, so one very cheap way to make one is to put a voltage divider on the speaker output and step it down to headphone voltages. The output impedance will be very high due to the divider, but it is cheaper than putting in a second amplifier for the headphones.