Before, you just played the game. Now when you play the game, these things fill your “inventory” out of thin air, and you can make transactions with them including buying them (to complete your collection out of FOMO, etc), of which Valve earns a cut out of them, again, out of thin air. Now scale that up to millions of users. Hmmm, it IS milking! Can’t expect any less from the House of Valve.