CPR for blood loss/gsw

I'm am a baby in the field of EMS (2.5 years basic in a very rural area, ER Tech, senior nursing student) so pardon me if this is a stupid question.

The title basically says it but let me elaborate. I have never had this call but hear about it (and of course it gets portrayed in movies)

In a case of massive blood loss and they arrest (I just came from the thread about the hockey player) why is cpr started and what are the chances of cpr being successful? If it's an uncontrolled/inadequately controlled bleed, it's seems like a lost cause and should simply be called "injuries incompatable with life" and left at that. For instance if someone is shot in the chest and they arrest, I would think they arrested because they simply ran out of blood or they were shot through the heart. Either way, cpr ain't fixing shit.

I get it, if they are on the OR table or you can definitively stop the bleed and administer blood/saline quick enough, it might be worth a shot.

Or is this something professionals are seen doing only in the movies and the rest of the time, lay people start compression without hope?

I know we should do everything we can to save people but as my AC likes to say, at some point, it's abuse of a corpse.