At the moment Apple is investing only in Siri voices making them better each year, while there has been no work on other voices for years. However Apple does not allow 3rd party apps to use Siri voices which artificially limits the level of accessibility that they can provide.
A friend has put an effort to create a petition to ask Apple to fix this. There is a good video and text there that fully explains the problem and how it can be easily fixed if Apple decides to. Please check it and sign it to make one step to the more accessible world:
$1UPDATE 2: Just for the record I have wrote to
$1 regarding this and they have answered that it will be considered by the development teams. So it is not impossible to unlock Siri voice just like some people claimed below.
DISAMBIGUATION UPDATE:
Thanks to a comment below I understand that this might be somewhat confusing to some users that may not understand some technical terms mentioned above properly.
To the most part this confusion is made by Apple as Apple uses term 'Siri' for two entirely different functions. First there is Siri the voice. You can go to the system Settings > Accessibility > Spoken Content > Voices and see that Siri is listed as just another voice, just like Alex or Samantha. The other thing is Siri the assistant which is a system app that handles conversational style requests from the user. Those are two entirely different things and if Apple allowed you to pick the custom voice for the assistant there would be virtually no connection between them.
This petition is about Siri the voice. You have paid to be able to use Siri the voice wherever you want including for example in Voice Dream Reader. But you can't as Apple considers this unsafe. The reason is that technically some app could use Siri the voice to trick you into talking with it and providing sensitive data if you think that it is Siri the assistant as you may not look into the screen but just hear the voice. However the privacy problem is not a novel thing and Apple allows apps to access dozens of privacy sensitive functions as long as they get approval by the user. There is no reason why this wouldn't be the case with Siri the voice. Again you have paid for it and you have the right to use it the way you want. And the fact that this limits accessibility makes it much more urgent.