explain to me why a Bible study cannot be a simple reading of the Bible
That's a great question. You should insist that your wife answer it.
The real answer is that studying the Bible alone would never lead a person to accept the numerous WT teachings, practices and doctrines.
But more importantly, it would not lead a person to profess obedience to the organization. Ultimately, it's about control. The WTBTS is a high-control, authoritarian religion. The Bible alone is insufficient to indoctrinate an individual into their belief system.