Seems the Scientific community is less certain it should be one system; " The Ganges River Basin is sometimes considered part of a larger river basin consisting of the nearby Brahmaputra and Meghna rivers. Known as the Ganges-Brahmaputra-Meghna (GBM) River Basin, it is one of the largest river systems in the world." [https://education.nationalgeographic.org/resource/ganges-river-basin]I think it's not being a major river, but more being the main river for a basin. For Mesopotamia and the Chao-Praya river, it's less clear on where it originates and where the exact headwaters are, so they got more than one river covering it vs the Ganges and Bhramatapura rivers, which one is clearly the main river.
It's described as a Transboundary River system when considered together, so both are equally valid as the main river