Why is soba not entirely buckwheat flour?

It isn't only that buckwheat flour doesn't have gluten (though that is true), but that it needs a binder, and using wheat flour is an easy methods of introducing one. In traditional (wheat) breads, the binder is gluten, which is formed as the dough is being worked - it is that formation which makes the mixture a dough, rather than a wet paste or grit. Other kinds of flours can bind to themselves in different ways - rice flour is used to make dumplings and mochi, even though it is gluten free, because the rice can stick to itself and be worked into a smooth paste.

Buckwheat flour doesn't produce its own binder in the same way. A dough made from just buckwheat flour and water will prefer to crumble, instead of come together in a dough. If you let the mix dry out, it will separate back into clumped up flour, if you add too much water, it will dissolve - since the flour particles aren't really sticking to each other. A pure buckwheat flour noodle therefore takes a lot of work or skill, to keep working the mix until it comes apart enough in the water and mechanical working to produce some kind of binder. The binder produced this way is weak, though, so the noodles are often still fragile, and prone to coming apart when agitated or disturbed, or when introduced to too much water (ie, when boiled).

So, yes, there are 100% buckwheat noodles made of buckwheat flour and water. There are probably buckwheat noodles that use the flour and a different kind of binder - adding milk or eggs would probably help bring the dough together, though it will also change the noodle. Adding flour is probably easier or makes less of a change, and wheat four is very often used in recipes either because it is very common, or else because it pairs well with the buckwheat (not sure which, I haven't tested it). But, adding any kind of binder should work to make a noodle.

Buckwheat flour is gluten free. The small amount of wheat flour gives it more stretch, and helps it stay together a bit better. For more information, check out this article.

As @margaio said, Buckwheat flour doesn't have gluten, but there exists 100% buckwheat noodles. It is very fragile, and breaks as soon as chopstick touches the noodle.