Three reasons.
First, reliability. Wireless cameras depend on Wi-Fi signals that can be disrupted by walls, appliances, weather, or a £20 jammer from the internet. A criminal can disable your entire system from outside your house without touching anything. Wired cameras can't be jammed. The connection is physical.
Second, power. Battery cameras have to conserve energy. They sleep most of the time and wake up when they detect motion. That means they miss everything between motion events. Wired cameras are always on, always watching. scOS needs that continuous feed to understand what's normal and catch what isn't.
Third, quality. Wireless cameras compress video aggressively to preserve battery and bandwidth. Wired cameras can push high-quality streams continuously without compromise.
Wireless cameras are convenient to install. Wired cameras are harder to defeat.