This is also true of Spanish and Portuguese but they are still considered different languages. If two people speaking different dailects couldn't understand each other they are surely different languages
To take a different perspective, calling them separate languages helps reinforce a nationalist agenda. It gives your citizenry another reason to feel separate from, and maybe superior to, other nations. Calling Spanish and Portuguese separate languages, instead of two Iberian dialects of modern Latin, helps reinforce the idea that Spain and Portugal are separate nations, and not just different regions of the same peninsula.
It's not really by default, more like by necessity. Almost all categorization schemes for human cultural artifacts are necessarily blurry at the edges, and just because someone has provided a definition that is useful for one purpose (the study of linguistic differences across cultures) does not mean that it is suited for another purpose (rhetorically implying solidarity or separation between two sets of people).
To be consistent with the linguistic facts, one should speak of the Chinese languages. You could also consistently speak of a Chinese language with dialects, and then also consider Romanian, French, Spanish, Portuguese different dialects of the Romance language; but that would redefine the terms from how they're currently commonly understood.