This is mostly because Rome itself was never a maritime power. While they had a navy, the reason Rome became Rome was off the strength of their Legions, and the refinements and constant changes of that field army. Indeed, even the Calvary, while initially being a army of the rich who could afford horses, ended up being handed over to mostly mercenary forces and after the Second Punic war the Numidians in particular.

At the end of the day, once Carthage had been defeated there just wasn't another power at sea who could challenge Rome. Rome controlled the entire Mediterranean outside of Egypt (and by the time of Augustus also Egypt). As such the navy didn't really have a job to do, they where mostly there as a glorified transport and supply chain most of the time. All of Rome's enemies where the barbarians on their landlocked borders, such as Gall, the Rhine, and Sassanian empire. As such the Romans had no reason to look to their navy, and even less reason to pay them the kinds of ridiculous sums of money that the legions proper would end up being paid in the late 3-5th century AD before the collapse of the entire economic policy, but that's another topic.