I would imagine because you could group on the fly with another device which cannot play those files, and dynamic grouping is a core function of the system. 

No other device in my system, just the Sonos Connect in my DAC amp… So I’m wondering if the digital signal, nevertheless, is treated by “Connect” ?

Sonos only supports maximum of 24 bits to 48 kHz

https://support.sonos.com/en-gb/article/supported-audio-formats-for-sonos-music-library

I know that ; you are talking about using analog outputs.

But my Connect serves as a streamer, digital outputs: it should not convert the signal, but let the DAC amp do it and therefore, not be limited for 96 khz

Nope, 

nope Sonos does not support anything higher than 24/48 period.

Edit .. a Bluesound NODE would do what you want.

Regardless of how many devices you personally have, Sonos’ core function is as a multi-room streamer and as such it does not support files which cannot be played by all units.

Try a Wiim Pro. £150 off Amazon. Doesn’t strangle like Sonos . Queuing facility not quite as good but they’re working on it .

They sound better than the connect with an outboard DAC . I couple mine with the Ifi Zen One Signature.

Sonos really need to up their game on this one and fast. 
(forget the Wiim for multi room though as it’s a pita)