Problem is, there is no enemy that can do that. Parthia and later the Sassanids at best threaten the eastern provinces, but at worst will simply seek to reclaim Achaemenid holdings, which would be very bad for Rome but not the empire all together.
The Garamantes and Nubians/Kushites weren't that much of a threat, at best any African polities could have raided and caused trouble for Egypt and Mauretania.
And the Kushans, Chinese and Indians were of course, never going to threaten Rome unless they had united under some eastern mega empire baring down on them from Arabia to Crimea. In which case, the Romans would likely lose. But no Eastern empire would have formed that could have marched westward and sustained itself. The logistics and administration would mean no east Asian or South Asian polity could threaten the west. I mean I guess you could have a Biblical "Kings of the East" where somehow all of Asia unites and marches westward-but barring ROB(or well divine) intervention, this isn't really possible.
So the empire goes through long stretches of time, fighting only Barbarians, putting down subject revolts or troublesome client monarchs, and of course fighting the Persians. But even then, that's a lot of legions not doing much. Hence civil wars.
Their just wasn't an existential thread to engender unity and prevent power struggles.
