Because Google never updated the app to support the Quest or released it on the Quest store, which is a different store than the Rift, and probably has different requirements.
You could try the Wooorld or Wander apps, though, as alternatives, which are on the Quest store.
You can use the MQ3 as a PCVR headset by plugging in a very fast USB 3 cable and in that case you can use the Rift version on the Quest to run Google Earth. I was also looking at this model
I know you can use a Quest Link cable to play some things on the PC store and almost included it, but I didn't see Quest on the supported platforms for the store page and assumed it needed to be on there explicitly if it was going to work.
I'm now cross-referencing it with Lone Echo, which I know works on Quest 2 and 3 since I played it recently, and it also only says Rift and Rift S for supported platforms, so yeah Google Earth probably still works on Quest 3 via a Link cable, especially since it still has new reviews as of January 2024 on there. Haven't tried it myself though.
You could try the Wooorld or Wander apps, though, as alternatives, which are on the Quest store.
Good example, though.