Its not about cameraX, its about the resolutions made available by the HAL (which is relayed ultimately through cameraX to the camera application).
The bottom line is that the HAL does not offer 1:1 aspect ratio.
4:3 ratio is the native ratio of the sensors.
16:9 and other similar ratios (2:1, etc.) are achieved by cropping at the HAL level.
Technically, the HAL could support 1:1, also through cropping, but its an uncommon ratio so they don't offer it.