The people who built them were the early Christians of Rome. You don’t know about the cult of martyrs, and the hold it has on the early Christians. Peter and Paul were martyrs, human sacrifices to the Roman gods. Because they were so prominent in the Church, their shrines attracted pilgrims from the earliest time, but especially after the final desecration of Jerusalem by Hadrian.
It is STILL alive and well!
Just LOOK at some of the responses here on FR~