Co-design workshops seek solutions to complex, multi-stakeholder issues. These ephemeral encounters bring together designers and uninitiated individuals who embark in a facilitated process that mobilizes a range of simplified design tools and methods. Despite co-design's benefits in terms of representation and acceptability, these workshops also come with limitations and often fall short of their intended goals. Proceeding from stylized facts informed by both our experience and the literature, this study investigates why co-design struggles at maintaining engagement and fails to consistently deliver innovative output regardless of the number of participants involved. Namely, we employ a modelbuilding strategy to illuminate the main knowledge dynamics during workshops and to highlight a constrained 'reactive expansion' mechanism that explains known co-design's shortcomings. Implications for workshop facilitation and planning are offered in closing.