The greed of a giant dragon is rooted in its bloodline, it's their nature. Even Muria couldn't warp this nature, as he also possessed these dragon traits himself.
However, perhaps due to the presence of Titan bloodline in him, his greed only targeted rare treasures and resources such as legendary metals and life tree sap. As for resources like elemental crystals which are "common", he didn't care.
"Settle down and finish the elemental crystals. Don't think about taking them out." Muria sternly stared at these heavily injured giant dragons, "The elemental crystals you earn for slaying the demons will be given to you by Cassio after we tally up."
What's the best way to motivate a dragon? It's money! Money, in other words, are all the valuable items and rare resources.
Nothing motivates a dragon more than this, from freshly-hatched baby dragons to ancient dragons that have weathered countless years. Young and old, they would all strive and fight for money.