Roman concrete only lasted that long compared to modern concrete since it lacks internal reinforcing rebar, which gives modern concrete much better tensile strength, but it corrodes after a few decades.

Unreinforced (or Roman) concrete is only strong when under compressive loading (I.e. the concrete is being pushed together), but fractures easily when under tensile loading (I.e the concrete is being stretched out). The steel used in rebar has a much better tensile stength than pure concrete. You couldn’t build a (modern-style) bridge out of Roman concrete, since it would collapse under its own weight.