Because too many R&D centres are in China they have to fly people there every day. They're using planes like a bus driving to another city. Planes add a lot of pollution in the upper atmosphere and using planes like a bus ride is not environmentally friendly.
You might not understand due to geography, and that many scientists/engineers/etc want to stay in their own country, that transportation by bus is not practical.
Also...we are not talking about commuters.