For the same reason that a 1000w
PAR can won't melt a 16awg extension
cord -- the
RMS draw through the cable is insufficient to actually damage it or cause a fire. That doesn't mean it's a good idea.
Of course, a typical 22AWG
instrument cable is intended for 1 amp or less, and will melt almost instantly with 40 amps. At some
point between those two
current values, the
conductor insulation will catch on fire.
Also, damping factor of the
amplifier/
loudspeaker system is influenced significantly by the
speaker cable.
If your monitors are run at fairly quiet levels, sure, a 22awg cable is probably not a hazard and does not audibly affect the
system performance.
If you are like me, and are sometimes running 4Ω
monitor circuits at 500w
RMS (measured drive
level, with amp into
limiter and signal hovering between -6dB and -3dB from 1000w peak limit) then you can turn to
Ohm's Law and find that your
monitor speaker cable is probably carrying around 8 amps of
current. You can then figure the temperature rise and resistance change of the 22AWG cable to find that the cable would ultimately be heated to about 130C, which would ignite the
insulation, but will not be hot enough to melt the
conductor itself, so if they don't short together and trip fault protection in the
amplifier, the conductors will sustain a fire.
This is how electrical fires happen. Using
instrument cables for loudspeakers is stupid. Why take a chance?