TL;DW: whereas modern construction uses rebar as a way to keep concrete from fracturing under tensile stress, the Romans made their constructions enormous so that the weight of the structure itself would compress the material and keep it from failing from tensile stress. Their monuments weren't built huge just because it cool, but also because it was practical. But large concrete constructions are both expensive and take years and years to cure, and depending on your concrete chemistry the strongest mixtures can also be much more difficult to work with.
Roman concrete is not like modern concrete. The process in modern concrete does not stop and eventually makes it too brittle and falls apart, Roman concrete does not do this and can last a very long time.
You start talking about rebar and other complexities when you want a shape that puts concrete in tension (where it's very weak), instead of compression.
Since it’s 2000 years old now.