It depends on what class of car design you look at. There are many with higher power than the 12V bridge amplifiers.
I think your premises are false therefore the conclusion is also. If you are the rare person that needs 6kW 100V generator for an audio system, add a 2nd alternator.
Here is one such IC for that market.
The PMP11769 reference design is a complete solution for a 700W automotive external amp. The boost converter provides 36.0V @ 10.0A continuous / 20.0A peak from an input voltage of 9-16V. It contains two premium Class-D audio amplifiers, which provide 2x 175W @ 4-Ω load (left & right channel) and 1x 350W @ 2-Ω (subwoofer).The design is well suited for car audio external amp applications due to its thin profile and high power output.
- Interleaved Synchronous Boost + Audio Amplifier
- Output 36.0V @ 10.0A (continuous) / 20.0A (peak)
- Free-Running switching frequency of 250 kHz CCM
- one TPA3251D2 in stereo mode (175W 4Ω )
- one TPA3251D2 in mono mode subwoofer (350W 2Ω)
- -12.0V @ 50mA) with LM2841X-Q, +3.3V @ 50mA) with TPS7A6633-Q
- ~> 95% efficient 3~10A
- 0.6% V overshoot 10A to 20A