Watching that drone failing to assimilate that dead 8472 was hilarious like a roomba trapped in a shoe box.
The Borg might not accept defeat well, but every time they attacked the federation they had a new plan generated form assimilating thousands if not millions of humans to advise the battle.
The Borg can't change their attack plan on 8472 because they have no new information to evolve their battle strategy because 8472 resists assimilation, which means that at some point that the Borg must realize that they shouldn't proceed without a new better attack strategy.
It's like asking why would America send it's army to a country with no oil?