“Better” is a bit of a misnomer. What is actually claimed is that the “development of the productive forces” - which is to say, the extent to which that economic formation can get stuff done - increases at each stage.

In the model, before Rome (slave society) would have come primitive communism. The Romans got more architecture done, which is enough for the model.

Pushing on, the slaves got some of the benefits of the architecture. They were treated like property, but got use out of the aqueducts and hypocausts and roads that wouldn’t have existed otherwise.

How would you decide if their overall situation is “better”? It’s something British imperialists like to try to do for their empire, but I’d be wary of even trying the exercise, myself.