All the text-to-speech software I've used has a generic sounding accent for the country (your choices are typically American, Australian, British, Canadian) but there's a lot more accents out there. The software isn't bad - it sounds realistic - but I wish it sounded more how I would like it to.

There's some software, e.g. Cepstral Dallas - https://www.cepstral.com/en/demos but it sounds too robotic to actually use and that voice isn't available for Linux so I only have it installed on my MacBook.

I guess a lot of developers at e.g. Apple live in CA so Siri is probably influenced by that.