That's mostly related to them being rather dumb across the connection to the charger, instead of having the charge controller integrated with the battery management system. Plus probably cheaper overall construction because a higher percentage is spent on just energy capacity from economy of scale at the pack level (more cells per pack: less overhead from other components when normalized per-cell). Doesn't help that they'd prefer to keep the grid voltage out of the metal framed vehicle, and there's economic incentives to regulate the charging voltage/current at the transformer directly without any intermediate voltage like USB.

Maybe using USB-PD signaling for the finely adjustable voltage modes (PPS/AVS) could help though, at least if USB-PD coding has reasonable range left in the protocol fields there to communicate the entire voltage and current range that such an e-vehicle charger would want. Though there's other readily suitable communication protocols to pick from if USB-PD isn't suitable.