Dialects are always in flux and there are plenty of languages out there they haven't conquered, so ... I'd guess they're just leaving it running to detect new speech or languages or dialets or ... personal data gathering or... ?
In fact, Google Voice on Android was supposed to make calls via data-only (no minutes usage) on a smartphone, but carriers would not allow Google to do that. Hence the strange random-number dialing which utilizes a 3 way call to make a 2 way call by default. Years later, they quietly allowed calls over data but by then GV was already on lifesupport.
The speech patterns also differ heavily, with Youtubers (usually) talking in a presentative manner and speaking very clearly.
I think the way people talk in private phone calls, as well as the call audio quality, are much more similar to that of someone speaking to an assistant.