To answer your first question, simply because it makes the process of defeating and assimilating target civilizations easier.

They’re a hive-mind collective of cybernetic drones that lack emotion and individuality. What would it care whether the civilizations it plans to assimilate are cowering in fear at their incredible might, or confidently defying them vowing victory and vengeance? It’s all the same once the assimilation’s complete.

The Borg’s modus operandi of walking up to the front door and announcing their intentions isn’t some expression of braggadocio or display of self-confidence born out of their record of unbroken conquests. They operate that way because their probability models and knowledge of their opponent’s capabilities lead them to conclude that no threat is posed.

However, as we’ve seen time and time again, their models were incorrect and our dear heroes have proven themselves quite resilient. The only reason, therefore, to continue with a direct assault would be to satisfy their own wounded pride. But as we know, they don’t have pride. So if, again, their goal is to assimilate earth, then maybe they should try slapping a few cloaking devices on some cubes, sneaking right up to earth’s atmosphere, and stabbing the federation in the back before it has a chance to respond. Mission accomplished.