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

The use of the XLR connector system has nothing at all to do with the application (electrical specification or the type of signal). Electrical signalling may be balanced or unbalanced or differential. It does not even specify the number of connector pins.

The assumption is that 3 pin connectors define the use of balanced signalling, with positive (pin 2) and negative (pin 3) being equal and opposite in polarity with respect to signal ground (pin 1), and symmetrical with identical phasing.

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. In fact, the use of coaxial connectors (i.e. RCA pin connectors, tip/sleeve, BNC et al), is fundamentally incorrect because it uses one of the signal-carrying conductors as the shield.

To sum it up, in the audio spectrum, the type of connector matters very little. The quality of connector matters a lot. The use of balanced electrical signalling, the use of differential signalling, for interconnects is preferred over unbalanced electrical signalling, but only where the signal origin is balanced (i.e. at the output stage).

Some of the best sounding components ever made use only RCA.   The fact that a component has XLR does not mean it's "better"  .   My last preamp, Zesto Leto had a full set of both RCA and XLR inputs and outputs.   Aside from the difference in gain it was hard to tell which format was being used.   They sounded identical 

Andrew Robinson uses only RCA connections. He is not a big fan of XLR ....

I only know me truly high end manufacturer who does’t - Spectral - and they d sign their system end to end  - you need their cables! Boulder requires XLR , and Ayre includ s it, DCS, as well.

Ayre’s Charlie Hanson was a big advocate of balanced connections. Ayre’s user manuals state that balanced connections are preferred. Of course Ayre’s components are fully balanced, so a balanced connection with them makes sense.