I don't design automobile alternators, so I can't say exactly what goes into the engineering decisions. However, here are some reasonable speculations.

Alternator efficiency is simply not a big deal in a car. The power the engine has to put out to move the car dwarfs what the alternator requires. If this tiny fraction of overall motor power were 1/3 less it wouldn't make much of a difference. Therefore efficiency can be traded away to get other more important parameters. Some of those probably include high reliability in a harsh environment, operation over a wide temperature range, and keep going while being splashed with water containing dirt and road salt and other crud. The volumes are very high, so keeping cost down must also be a major desire.

First, look at the cost of a 90% effecient generator of the same power compared to a car alternator. I expect it will be several times more. Then try operating the high-efficiency generator in a harsh environment like under the hood of a car and see how long it survives. Car alternators routinely survive this for 10-15 years. The high end efficient generator that cost several times more probably won't last a month in bad conditions.

It's all about what's really important and making the appropriate engineering tradeoffs in the design.