This is exactly the same for me. i already own a lot of music. Why doesn’t it allow me to recognise “My Library” and simply play the music from there as the Sonos app does?

I’d say it’s a combination of how the technological works and a combination of Google wanting to support their music subscription and ecosystem in general.

Whenever you make a voice command, the command is processed in the cloud on Google’s servers.  The server validates the command, making sure that it knows what music you want to play and on what speakers.  If you don’t have a music service Google supports, or it doesn’t recognize the music or speaker you selected, then you’ll get an error right here.  If everything validates, then the command is passed to Sonos cloud service, which then passes it on to your speaker to play.    So  understanding this, you can see how Google is preventing access to play your local library or some other source before it even gets back to Sonos.  This is not unique to Google, as Amazon operates the same way.

The system/process could have been designed differently, where validation occurs on Sonos servers, which could be aware of your local library and all your streaming services.  The technical downside to that it puts a lot more processsing burden on Sonos servers and creates some potential security concerns, since the Sonos clould would need to index your local library and generally have more access to your local network.  That would be a big concern for a lot of users.  Although Sonos probably would like to give it’s customers greater access in theory, since they don’t make money off of any subscriptions, the technical burden on them would be greater,  while placing the burden on Google or Amazon is minimal since they already handle this function with their own voice assistant products.

As you stated already,  the decision was surely not just technical, as Google and Amazon probably would have fought integration with Sonos harder if it enabled users to use their infrastructure without using their services.  I personally don’t see this changing unless there is a signficant change in the market where more people are using local libraries and/or there’s a willingness to pay for additional features.  Perhaps a new voice assistant will come along, one that doesn’t have it’s own music service tied to it, then will change how things can be done.  

IIRC you can create a Plex instance which will allow voice commands.  

I have Alexa and Spotify as well as a large “My Music” library on my NAS.  Whilst I occasionally ask Alexa to play music, i would say 90% of the time i revert to the app and chose myself.

I use napster which is half the price of the other big services and I also have a large collection of alternative music on my pc. Ive been a long term user of napster so have no inclination to change yet the response when asked to incorporate napster or local libraries with alexa has always been the same - "we are currently not able to do this, will let you know blah blah, the infrastructure is not able to, blah blah, security concerns, blah blah, its the providers fault, blah blah."

Regardless of who or what is at fault, it is frustrating to the end user who just wants to say "alexa, play some music".

I now use a third party skill called my speaker, i have to say "alexa, ask my speaker to play dire straits", an extra 4 words but it works. If its set to play from napster, it works. If its set to play from my library it works. If a third party can implement something then surely a large team like sonos could do it. 

As far as I’m aware both Google Assistant and Amazon Alexa will recognise and work with their streaming competitors as music sources (e.g. Spotify premium )  without any revenue benefit.

Unless Google/Amazon log the use of all music streaming services activated by their voice controllers and charge other providers an agreed monthly percentage ?  Or am I being too devious ?

I should have been more clear.  “Their services” was about the services Amazon offers through their echo device, whether it be Amazon music, Spotify, Pandora, Apple, etc.  I don’t know if licensing fees are collected, or if Amazon/Google allows this in order bring in more customers, avoid anti-trust concerns, or some other strategic reason.  I don’t think it’s all about revenue exactly, but control of the ecosystem.   And I said before, tech development wise, this is the easier path.

Regardless, it is what it is.