ULA didn't even exist 50 years ago. Boeing bought their launchers division from Douglas. Atlas was originally built by Convair. People die and institutional knowledge dies with them. Then again Delta IV and Atlas V have little to do with the original rockets. For example they use isogrid manufacturing methods which weren't in common use at the time.
Also you can get a pretty good ideas of the reliability of a rocket with ten launches. Even 3 launches can be good enough for most purposes. Most accidents with a rocket happen in the initial batch of launches and once you get past that hump the rocket is usually pretty reliable.
There are models for this. Given Falcon 9's past launch record it probably has a reliability rate of 90% or more.
If their reliability rate was poor the insurance rate for the satellites they launch would go though the roof and no comsat operator would use them regardless of how cheap the launch price was. But it seems their launch manifest is quite full with orders so it seems the insurance companies disagree with your perspective.