Yes, I'd imagine it's because of the number of treasure chests. Coincidentally, a few years ago I came to the same conclusion.

But I think any of the starter zones or smaller DLC zones should be more or less equally "best" as far as the number of chests and their spawning rates.

(I'm assuming that treasure chests are like resource nodes-- that a given zone has a specific number of possible spawning locations, and the game tries to keep a specific number of treasure chests available at any time, so once an existing treasure chest has despawned on its own or been fully looted the game will spawn a new treasure chest at some random location to replace the old one.)

For the last month or so I've taken to farming treasure chests in Hew's Bane whenever there's an endeavor to loot chests or pick locks, because I haven't collected all of the set gear from that zone yet. The starter zones have only one set, so unless you're specifically wanting to farm Trainee gear you're probably better off farming treasure chests in whichever zone (large or small) has sets you haven't finished collecting yet or are the most interested in.

Public dungeons and delves can also be good places to farm treasure chests in, but some of them seem to contain a lot more treasure chests than others. For instance, I was doing the public dungeon in Reaper's March for the Mages Guild daily for the public dungeon group event endeavor and decided to kill two birds with one stone by farming treasure chests there for the lock-picking endeavor, but for whatever reason I couldn't find a single treasure chest! In contrast, the public dungeon in Malabal Tor seems to be a much more reliable source of treasure chests.