"trying"

That is the point, though. An interesting neighbour to conquer is one which looks wealthy enough (no sense in conquering cold northern marshes full of mosquitos), and is weak enough so that you may actually conquer him and hold the territory without overextending yourself.

Most Roman neighbours by 1 AD would not pass the first test. The Persians would absolutely ace the first test, but they were equals to the Romans when it came to military prowess, and a danger to be reckoned with. As a result, the Eastern border was, overall, a source of weakness rather than profit.

Modern MBAs would probably call the Roman-Persian border a "cost center".