Legal issues. Dash cams aren't allowed universally, and even when they are they tend to have restrictions (how it's mounted, how footage is handled, etc). They've just decided dealing with this pain point is not worth the purchase conversions since users can just add one themselves.
There are some brands that have them, usually as an additional package depending on where you are. Like Toyota's Genuine Dash in Canada, Tesla Dashcam, BMW Car Eye, etc.
All you need is a toggle in the settings that is off by default. They also already deal with regional law differences regarding speed camera warnings, so it isn't an insane idea for them to maintain a list of countries where the feature is blocked (although that's not the solution I would prefer)
There are some brands that have them, usually as an additional package depending on where you are. Like Toyota's Genuine Dash in Canada, Tesla Dashcam, BMW Car Eye, etc.