The Borg can only acquire superior technology when it is being defended by inferior (military) technology.
A Borg Cube might be repelled by a powerful alien Star Ship, but they could also figure out the tech in that Star Ship and how to counter it's shields and weapons, by bullying a tiny shuttle underpinned by identical technology, and taking what they need from junior.
The Borg are also not that sneaky.
They believe that everyone wants to be Borg and that they are being helpful and generous by assimilating the galaxy.
The borg can't acquire superior technology because they wouldn't know how to use it, they' wouldn't know how it works. If something is based on lasers (like a DVD player for example) and if you don't know about lasers, how would you know how to use it?