Amber being fossilized tree sap, true. Some estimates I could find online indicated that the transition from sap to amber takes well in the millions of years, a few figures citing 2-10 million.
So while Amber does come from trees, and the desert has no trees, its very much likely that once upon a time, that desert WAS once a forest like that, while the currently thriving jungle is unlikely to have been there for such along period of time, and thus, amber from said contemporary jungle is unlikely to be present.
Of course, this is also Terraria, where drinking water while drowning heals you, and legendary metal can be used to make bubblegum, so what do I know.
