Unless they were as decimated by the Burn as everyone else.
There's no reason to think they were.
The Borg have been using an unknown power source for their ships... but nothing in live action series suggests use of Dilithium and M/AM.
Although its possible that once VOY crippled the Borg in 'Endgame'... it was something from which they couldn't really recover easily.
After all, the Queen was destroyed along with the main Unicomplex in the DQ (the very heart and brains of the Borg).
Its possible all or most Borg everywhere became disconnected from the Hive mind once this was done and the TW network was obliterated (and I suspect it might be possible this is how the Artifact in ST: Picard was captured and studied by Romulans).
That was addressed on the show, the Borg don't make an effort to "rescue" a ship after it's been disconnected from the Collective, which we've seen throughout TNG and Voyager is indeed the case.
Seven of Nine stated otherwise and we saw in one of her memories that the Borg DID send a rescue ship. I would imagine the reason the Borg didn't come back for some of the disconnected ships was because they couldn't pinpoint their exact coordinates, or the risk of expending resources to send rescue vessels was deemed too great.
In the case of a cube disabled in the Nekrit Expanse, it was hit by a massive electrical surge. The surge likely disconnected the cube from the Hive immediately and it could have been enough of an deterrent to make the Borg think twice about sending another ship there (which could end up in the same situation).
In case of the Borg kids... they were simply speaking rejected by the Hive mind and because of the virus which disabled the that cube... it was simply detached like a deseased limb left to rot. Space is huge/vast.
I guess the Borg could have sent a ship to destroy that one... but with the original virus spreading through the cube via Icheb, its possible the Borg didn't want to risk the virus being replicated and sent over to another ship.
And in the case of the Vinculuum episode... well, I just guess a Borg ship never had an opportunity to come by yet... or the Hive mind would have discovered the initial plans for disrupting the collective when they assimilated the said species (as I suspect the pathogen would take time in attacking the cube which assimilated them).
So, again, it may have been seen like a diseased limb
There ARE explanations behind Borg's actions... i