Because your assumption that balanced is better is not correct.
If you’re saying, they should offer both, that is not without compromise. Simpler signal path is superior.
If the balanced connection adheres to AES48 then it is better in every way. If the connection does not support AES48 then its a crap-shoot!
If it is a short cable run then un-balanced is fine and the shorter the better.
This is a common myth. The advantage of a balanced connection is there even if the cable is only 6 inches long.
Some applications simply do not adhere to the concept of balanced signalling (i.e. phono cartridges), but do work well at maintaining an electrically consistent path from cartridge to phono stage.
FWIW all phono cartridges (with the exception of some older Decca cartridges which only have 3 pins) are the definition of a balanced source. That is why when run single-ended, a ground wire is included since the ground is independent of the source wiring and so will introduce a buzz if not grounded.
As for the "long run" cable scenario. my preamp is 18 feet away from my amps. I have been told by many sources this distance is nothing for an RCA cable. Now Hegel on the other hand, suggests the use of high quality XLR cables is the way to go. I have tried both and must say I hear no difference in sound. Tempest in a tea cup IMHO.
10 feet is pushing it for any RCA connection! Otherwise you can get into audible high frequency roll off depending on the input impedance that is being driven- the higher the impedance (tube amplifiers take note) the worse this problem is. For really pricy cables attempting to get around this problem, read on...
What really matters is if the audio equipment has a “true balanced” design. Meaning that there is identical circuits for the positive and negative signals. This means twice the parts and cost pretty much. Twice as many tubes or transistors and a higher current power supply to power it all.
This statement is often false. If the circuit is balanced differential there are not twice as many parts! For example any kind of equalization can be done in the differential domain using the same number of parts as a single-ended EQ circuit. The only place that requires duplicate (balanced) wiring is when the circuit is being processed by a volume control or being switched.
You don’t have to have balanced circuitry to run a balanced line properly. The venerable Ampex 351 tape electronics are entirely single-ended, using transformers at the input and output to receive and send balanced signals.
The advantage of running differential balanced internal circuitry is you get so much more immunity to power supply problems as any power supply noise is common mode so does not get amplified. This results in not only less noise but less intermodulations since power supply noise intermodulates with the signal due to non-linearities in the tube or transistor involved.
Internally balanced operation is also inherently lower distortion because even ordered harmonics are cancelled from stage to stage. Because distortion isn’t compounded as much thruout the circuit, you get a more transparent sound since distortion obscures detail.
Some will argue that the remaining distortion is odd ordered, but the ear treats the 3rd harmonic in much the same way that it does the 2nd (its innocuous) and its able to mask succeeding harmonics quite well. But when you look at the distortion spectra you see the succeeding harmonics dropping off at a faster rate than with a single-ended circuit. High end audio is really about getting as close to the sound of the music as possible so this is a real bonus!
Another enormous reason to go balanced is the interconnect cables. This is not all that hard to understand. In the old days before Robert Fulton founded the high end exotic cable industry (mid 1970s), the only way you could get signals from point A to B without the cable interacting with the signal was by using balanced lines. This is how microphone signals were able to travel in cables insulated with rubber and PVC and not get all messed up like you see in high end audio if the exotic materials are not used. We all have recordings that were done in this manner!
Put another way, in high end audio the technique has been to throw money at the cable problem and if you heard a difference then the money was deemed ’well spent’. But imagine an exotic cable system where the equipment that drives and receives the signal does all the heavy lifting, allowing the cable to be inexpensive with no downside. That’s the balanced line system, that’s why there is a standard for it (AES48 combined with low impedance operation) and that’s why its been used by the recording/broadcast industry as soon as it was figured out 70 years ago. Its simply more musical, more neutral.
Again, if you find yourself disagreeing with any of this, its probably because the equipment you heard didn’t support the standard. This throws out the baby with the bath. ...FWIW We were the first company making balanced line equipment for home stereo and it was really hard to get going with it because it was for home audio not pro audio. When we finally started seeing competitors jump in, we cringed seeing so many of them either ignore the standards or seemingly were ignorant of them. As a result we also started seeing high end audio balanced interconnect, which is really ridiculous: the whole point of balanced operation is to rid yourself of cable interactions (and ground loops)!! If you’ve ever auditioned interconnect cables and heard a difference you know what I’m talking about.