At this level of high(er) end audio why not offer balanced connections?

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.  

Balanced is preferable for very long cable runs such as mono-block amplifiers that will be located next to speakers and away from the pre-amp and sources. Unless the component is truly balanced their is no point. 

As @carlsbad said you are making assumptions. If it is a short cable run then un-balanced is fine and the shorter the better. 

OP examples?

This is an @atmasphere , question.

He was one of the first designers to offer equipment that used the AES 48 standard.

Long story short, designing equipment that not only offers XLR, but adheres to the AES standard, makes it challenging. Many offer XLR connectivity, but do not use the AES standard-which makes the XLR connection subject to cable influence.

Okay, I should probably back up even further. 

The AES 48 standard insulates the cable from introducing 'cable artifacts'-or for a better phrase, 'cable sound'. This standard has been utilized by recording studios for decades. And, understandably so, considering that equipment is spaced over quite long distances.

So, when a manufacturer offers XLR/balanced, the equipment should adhere to the AES standard in order for the cable to introduce the minimal amount of 'coloration'.

Unfortunately, doing so makes it a bit of a design problem, albeit with additional cost.

Hopefully, Ralph(Atmasphere) will chime in.

Bob

@jerryg123 Yes, I'm speaking of those with their amp in the same room as their preamp.