Home Stereo vs. Music gear power rating
It is not about the wattage of the amplifier, which simply rates how much power it can give out (or consume), it tells nothing how the watts are actually being used.
For instance, your guitar amplifier might have a sensitive 4-ohm speaker that pushes out 90dB at 1W measured at 1m, and your home system might have a insensitive 8-ohm speaker that pushes out 80dB at 1W measured at 1m. Therefore you need 10W, or 10 times more power, to reach same 90dB volume level. For comparison, 80dB is approximately the sound level of one lawnmower.
The guitar amp may also be rated to allow for higher distortion than your Hi-Fi set for the same wattage.
Also, guitar amps typically effects like distortion, overdrive, compression etc which can make the audio sound louder. It's basically gives a perception of being louder, much like a sound of someone shouting but played back at low volume level, than someone whispering at high volume level, even though a decibel meter indicates the levels are equal.