[Kirby64]

No it's not, only in a practical sense. If you truly had 'arm thick cables', you could definitely charge a 48V battery just as fast. Practically speaking, though, you don't do this because every becomes so unmanageable that you can't build a charger, bus bars, etc, that would be able to match the charging speed.

[jandrese]

The problem isn't the cables in your car, it is the cable between the DC fast charger's transformer and your car. They are already thermally limited, which is why you need higher voltages to support faster charging.

[Kirby64]

Like I said, this assumes you use ridiculous cabling and bus bars. You could design something that handles this, it would just be wildly impractically large and cost way too much money.

Also, the problem is definitely also the cables in your car. Moving to 48V would mean amperage would increase by 10-20x, which would mean cabling thickness would have to increase substantially.