There are some reasons why you might not want to do that, or why that might be inconvenient. You could end up with weird voltages floating around and interfering with other parts of the circuit, or you could end up with an unexpected ground connection going through a chip somewhere. Basically, circuits (and ICs) are often designed with the assumption that ground is always ground. Sometimes you can power a chip just by supplying a voltage to the signal pins. This usually ends badly.

Grounds are often redundant and built into the chassis (of both the component and the laptop), routing back to the power supply. This is to create a sort of "faraday cage" that protects it against harmful electromagnetic interference and discharge (e.g. static electricity). It would be almost impossible to separate it from ground completely.