The book Dreamland[1] chronicles a lot of this history... but a belief in science means that if scientific studies challenge some of your preexisting beliefs, you accept that evidence.
The doctors were getting shown a whole host of studies which all said that there was only a minor addiction risk. You need to be able to put trust into what the studies are showing.
I'm shocked to the point of disbelief on this as well, this does not pass the smell test for me. Anybody, especially doctors, should have been well aware of the risks, this was not a surprise to anybody with as much as a toe in the public health sector.
[1] https://www.amazon.com/Dreamland-True-Americas-Opiate-Epidem...