It’s remnants from a time where security wasn’t a concern. The original intent of the From: field in email was that it’s definitive, but now it’s just a legacy field that many systems ignore because it’s fakeable.
Possibly, but I'd wager more that it was due to Bell owning the entire network, so they knew that any "from" numbers were correct. If I'm reading Wikipedia right, SS7, which is still supported today, was created all the way back in 1975![0] Bell wasn't broken up until 1982.[1]
You're correct, but back in the day, all nodes were trusted nodes, so would have been a lot of overhead to authenticate all this stuff. Hasty regulatory oversight in a fledgling industry led to the current situation.
STIR/SHAKEN actually has the potential to do things correctly, as a call Digital Attestation Certificate has to be supplied... but telcos make quite a bit of money off of scam callers so don't expect them to move quickly, and I'd expect them to implement it in the absolutely poorest way possible.
Similar to the original SMTP implementation, it was designed in an era where folks often assumed that only "trusted parties" had access to the network backbone (whether that network is circuit-switched or packet-switched).
