So long as the two countries were divided, the borders were policed by neither Scotland nor England; consequently, Reivers could reive in both countries and hide in the borders.
The Borders also provided refuge for Scots families who had been outlawed... as was often the case. These families tended to be politically troublesome as well as criminal, so inevitably they gave allegiance to no-one and stole from everyone.
Brian Moffatt has researched and recorded everything there is to know about the Reivers over on the West side of Northumberland. His website is a monumental - ongoing - work that gives jaw-dropping insight into the Reivers and the debatable lands:
https://fallingangelslosthighways.bl.../?view=classic
Stealing cattle in Scotland and driving it into England... and vice-versa, was simply too good an opportunity to ignore. Stealing weapons from both countries was also a good idea and consequently led to the enormous variety of swords used.
Here on Tyneside we have an expression: "The Wilds of Wannie" meaning beyond civilisation. This comes from the Wansbeck River and hills running West to East across Northumberland about 20 miles north of Newcastle. Once you crossed over, you were in territory that was not policed and obviously dangerous; even after unification it was still not safe.