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.