The stock alternator and battery meet the electrical needs of your car’s stock features, and that’s about it.
The draw from a big* audio amplifier – combined with regular vehicle features – runs your battery down faster than the alternator can charge it. This causes the voltage to drop.
Voltage drops damage amps.
*A “big” amp is anything over 1000 W RMS. Smaller amps are often able to run on a stock electrical system, but upgraded charging never hurts!