Because one of the basis of modern (well at least for 30 years) networking is the concept of layers. The role of a VPN is to encapsulate network traffic to allow to remote sub-networks connected through en uncontrolled network behave as is they were locally connected. No more no less.
But there are other projects that allow to hide a network stream in a tunnel encapsulated in a HTTP conversation. Some example of those have been discussed in that SO question.
Technically, it is possible to encapsulate a full VPN flow inside a HTTP(S) tunnel. But I must admit that I do not know a full out of the box solution. Maybe because there are not that much acceptable use cases for it: VPN are highly used to joint corporate networks so you can find plenty of references of that. But HTTP tunneling sounds like I know you do not want me to do that so I'll just hide it. And then the problem instantly passes from the technical side to the legal one...
And even if the traffic might use correct HTTP(S) requests and responses, it is likely to have slight differences in request sizes and delays that allow heuristic methods to discover it. Once it is detected it can be a matter of hours for the police to arrive. I would never use that in a country like China!
For temporary residents, a solution would be to setup a private VPN before going to China, and then simply use it. If it only carries little traffic it should not be banned.