Do you perhaps have a browser setting that nukes cookies/session data by any chance? Or perhaps use a VPN that might be tripping some sort of account protection mechanism?
What I do have, and I expect is relevant: frequent ~weeklong gaps where I don't access GitHub at all in this browser profile. I assume there's some medium-lived token that's refreshed when you access the site.
This feeling could also be exasperated though since while I only use a personal GitHub account, I access it frequently from the browser and app on numerous devices.
I can definitively say though that I need to login more than twice a year on any one device.