The political will doesn't exist to make it happen, and given the physical geography of the border region, it would take a lot of cash and political will.
The border between India and China is some of the hardest terrain in the world. It is a nearly unbroken line of mountains. In the areas in which the border is undisputed, it is because there is a clear ridge of mountains which mark the border. Connecting by an overland route would be very challenging. Building a tunnel would be very costly.
The area immediately north of India is Tibet. It is sparsely populated and there is an ongoing independence movement (which is supported by India). The route from the densely populated Ganges valley to the Eastern Lowlands of China would be a very long one. It is easier and probably cheaper to fly. China has little interest in building links between Tibet and India.
I don't know much about the local people. I suspect that many are semi-nomadic or tribal herders. I doubt they would find much use for a road.
For trade, there is a much simpler way: put on a boat and go round Malaysia to Shanghai or Hong Kong. It may be slower, but it doesn't require major infrastructure investment.
Or, if you really want to go overland, and can sort out the paperwork, you can go via Nepal or Myanmar.
India and China have both proved that they are capable of major works. If the will was there, a road could be built if both sides wanted it to prove something. Right now, though they both want no road to prove that they are unwilling to compromise on the border.