Cost. I know some professional amplifiers that have balanced inputs, but internally short pin 3 to ground. It's kind of a waste, but it's a necessary convenience feature there.
In the residential world, you could add an XLR input, and do the same thing, but you won't get any benefit from it unless you actually have a balanced input stage, and even then, whether it's a benefit or not would be debatable. It would involve more circuit complexity.
By leaving the XLR connector out they can save money, keep the price a little lower, and leave it up to you if you must have XLR. I know of very few preamps that don't offer RCA outputs, so most of the time this makes sense.
If you need XLR to minimize noise due a longer run, you'll probably want to use a transformer at the amp end, but only very long runs prone to noise, or near powerful sources of noise such as a radio tower would benefit.
Best,
Erik
In the residential world, you could add an XLR input, and do the same thing, but you won't get any benefit from it unless you actually have a balanced input stage, and even then, whether it's a benefit or not would be debatable. It would involve more circuit complexity.
By leaving the XLR connector out they can save money, keep the price a little lower, and leave it up to you if you must have XLR. I know of very few preamps that don't offer RCA outputs, so most of the time this makes sense.
If you need XLR to minimize noise due a longer run, you'll probably want to use a transformer at the amp end, but only very long runs prone to noise, or near powerful sources of noise such as a radio tower would benefit.
Best,
Erik