Why create an artificial one when you already have the real one?
Now for a more general (and useful) answer: artificial (or more properly "active") center taps or virtual grounds, have one problem: they must source or sink *all* the current pasing through the load, same as a "real" ground would.
It's practical in a preamp , or in a lot of 9V powered Guitar pedals, small battery powered mic mixers and such, because since they basically process *signal* (meaning voltage into high impedance loads), current is very low.
So, say, a humble Op Amp which can provide 5 to 10 mA on demand, is adequate.
But in a Power amp, we are talking big Amperes.
A 100W into 4 ohms amp supplies 7A peak current into the load (and into the speaker ground return wire) all day long, and such return should be able to flawlessly pass 10A, to have a slight margin.
Very easy task for 8 inches of #18 (1 mm) wire which cost a few cents and at most will drop a few millivolts.
But if you want to create an active ground, it will need to pass the same current, will drop much more voltage (at least 1 or 2 diode drops at the junctions) and in general will need semiconductors as beefy as the power output ones.
So it's not that it's technically or physically impossible to do it, it's just that it's inefficient and expensive compared to the alternative.
While in, say, a preamp or pedal, 1/2 TL072 (or even a 741) , 2 resistors and 2 100uF caps will add to less than 1 U$.