We sell alarm services. Most people only arm their systems in away mode. Stay mode is rare, and is almost entirely by single women or female spouses with male spouses not home. Standard security has its limitations and even if armed, has openings unless someone really is paranoid and layers a lot of devices. Standard door/window contacts only trip if opened, so if you just kick in glass and step through the hole, that sensor isn’t tripped. Glass break sensors cover a pretty small area, and most people don’t spring for having them cover every piece of glass in the home. It’s entirely possible that the system was armed, but this guy used a hammer and knocked out a bedroom window that was not in range of a glass break sensor and crawled through the opening instead of popping a pane and unlocking/opening the window to get in.
Houses are really easy to break into, and even the best security system isn’t going to stop someone with intent.