[Roman mindset]
You don't draw borders based on where mice would go, right? Or where deer would. Why would you do it with barbarians? It's literally a non-issue, they're more like local fauna than actual = Roman people!
Plus those barbarians do all sorts of disgusting stuff. Like, have you heard that they're too dumb to use a strigil? They can't clean their own bodies properly, even dogs can but they can't! Instead they mix lard with ashes and smear that shit over their bodies, it reminds me pigs that rolled on mud.
...wait, Celts were the ones with the lard and ash thing? Not Picts? Do I look like a zoologist? They work the same once you enslave them and throw them in a garum factory, stop fussing over spurious distinctions!
[/Roman mindset]