Precisely. Publications require data. Data costs time/money/brains. Grants/promotions/tenure/survival cost publications. It's not hard to see that data serves a

Precisely. Publications require data. Data costs time/money/brains. Grants/promotions/tenure/survival cost publications. It's not hard to see that data serves a role as currency, and there is little incentive to give it away as long as publications are what puts bread on the table. Additionally, the discussion over shared data is usually a cry to improve reproducibility. In this publication-centric world, one can't publish a paper that is just "we reproduced an existing paper", so outsiders look to "poach" new applications or findings from the data.