Oddly, it's not. That's where OP is coming from. "Treasure trove" comes ultimately from Latin via French (or at least, some language fragments the Normans brought over). The "trove" means "found", so it's "found treasure". That's why in the original (pre-English) phrase, the word order is backwards: "trove" is the adjective, "treasure" is the noun, and it follows the appropriate French/Latin word order. It was pulled directly into English without reordering (common for borrowed phrases). Eventually, "trove" (which had no English meaning at all) became a synonym (a shortening) for "treasure trove".
So by etymology, "trove" was originally an adjective. However, it means nothing in English. The phrase "treasure trove" is a noun phrase all by itself that can't really be broken into parts.