Alright, so everyone knows about the absence of Semiconductor chips in the fallout universe. Look at all the vacuum tube Radiation King TVS! But why? Why didn't they stumble upon something similar at one point?

There are circuit boards in the game. Go shoot some Turrets and loot them. (at least in Fallout 4)

In the early days of IC's, higher power applications still used Tubes, which is why TVs had Tubes up until the late 70's even after transistor radios had been out since the 60's. Radio stations even used tubes for a longer period of time after that in their high power transmitters.

Fallout takes place in an alternate timeline that split around the 1950's, so not everything we accomplished in our world was accomplished in theirs. Besides, they had Fusion powered cars, we still don't have a viable fusion reactor in our world.