The difference is that they didn't have rebar. And so they built gravity stable structures. Heavy and costly as fuck.
A modern steel and concrete structure is much lighter and much cheaper to produce.
It does mean a nodern structure doesn't last as long but also the roman stuff we see is what survived the test of time, not what crumbled.
Roman concrete is special because it is much more self-healing than modern concrete, and thus more durable.
However, that comes at the cost of being much less strong, set much slower and require rare ingredients. Roman concrete also doesn’t play nice with steel reinforcement.
Modern concrete is more uniform in mix, and thus it doesn't leave uncured portions.