Concrete is strong in compression but has almost zero strength in tension. The rebars that we see in modern concrete are to add strength in tension.
Buildings such as the Pantheon are designed such that the concrete is always in compression. The dome is a perfect example. Thus reinforcement would have been redundant.
But they were still brilliant engineers.
One wonders how many workers (probably slaves) died making that dome…. As well as other Roman structures, like the Colosseum, etc..