In every single high-powered car audio system I've seen, in person and on the internet, the amplifier has been run off of the main 12 volt power system.
Such systems feature multiple parallel runs of heavy-gauge (Commonly 2 AWG or lower) wire between the battery and amplifiers and several alternators in parallel to supply the required current, which is in the hundreds-of-amps range. For a 5KW system, the maximum current would be around 400 amps.
Additionally, modern class D subwoofer amplifiers often have easily 2/3 of the internal space dedicated to converting the ultra-high-current 12 volt input up to a higher voltage, typically 70-100 volts, to drive the subwoofers.
Instead of using the 12v and boosting it, an alternative approach would be to have the alternators wired in series or using higher turn count windings to generate a higher voltage (70-100 volts as above) and run the output stage directly off of that. It would be a much simpler setup, as you could omit the voltage booster section of the amplifier and would only need the output stage and signal processing.
Why isn't the latter setup used in high-power car audio systems?
[Edit] For clarification, I am not concerned with "normal" (~1kw or less) car audio setups. I'm asking about competition builds where they are using extremely high amounts of power and are so weighted down with equipment that their primary purpose is to make noise and set SPL records, not be everyday commuting vehicles.