Wow....I just assumed that it would have BSM AND rear cross traffic alert. I was under the impression it was the most technologically-advanced thing on the road.
To clarify, there is BSM, just that the UI is not particularly good as it only shows lines in the visualization.
See page 110 for how it looks:
https://www.tesla.com/sites/default/files/model_3_owners_manual_north_america_en.pdf
It also has an audible alert (see page 111), but only when it detects that a collision is imminent (it's not like other systems that might alert regardless):
"Blind Spot Collision Warning Chime
If you want a chime to sound when a vehicle is in your blind spot and a possible collision is detected, touch Controls > Autopilot > Blind Spot Collision Warning Chime."
I mean...my bad for nor asking, but why?
As discussed above, for BSM, Tesla can do better if they choose, however at the moment they don't for whatever reason.
As for rear cross traffic alert, it doesn't have the sensors for it, so doesn't have it at all (usually there are radar sensors mounted to the two rear corners of the vehicle for it). Other cars use the same radar sensors for blind spot monitoring, so they have both automatically (they go hand-in-hand). Example below from Continental:
https://www.continental-automotive....3007b4031/Rear-Cross-Traffic-Alert-EN.pdf.pdf
Tesla's blind spot monitoring uses the rear facing side marker cameras instead, which are blocked when parking, so can't do the same function in terms of monitoring rear cross traffic.
Tesla's autopilot page may help you understand the sensors better (click the "Learn more about Camera Views, Radar and Ultrasonics" to see more details about the FOV of the cameras).
Autopilot