Before airplanes, the easiest way to get from central Asia south to central India was to ride east to the freaking Pacific and take a boat.
It is THAT isolated from central Asia. (Less so from west Asia because the mountains and plateau only go so far west.)
And the reason it is isolated is the Tibetan plateau, whose southern edge is the Himalayan mountain range. There's another mountain range west of them as well.
The whole area is so unbelievably inaccessible that it is actually easier to travel to the south pole than it is to get to the center of the Tibetan plateau.
It is huge, incredibly high in altitude, dry as the desert that it is, almost completely unpopulated, and surrounded by the highest walls on Earth.
Understand that these aren't just mountain ranges; they're walls....