Electrical – In automotive electronics ECUs, why is an external EPROM IC often used instead of the MCU’s internal EPROM

A typical truck engine might produce 200-400kW [...]

That is mechanical power. Electrical power is generated in the alternator, and thouse typically generate power in the order of 1 kW, as that is enough to power the lights and charge the battery etc.

The dedicated power generators are also more fuel-efficient, and can produce higher voltages - a big truck operates at only 24 Volts. Light and sound installations in the multi-kW range will usually need 110 or 230 Volts to operate.

EDIT:

Wouldn't it be simpler and more efficient to upgrade the original vehicle's alternator

It would be simpler only when there is space available for a bigger alternator and when there is a bigger one actually available.

But it would never be more efficent in fuel: The generators engine is highly optimized for its task (hopefully), and will be operated at or near its point of maximum efficiency. It will also be a lot smaller, as you need only a few kW instead of 100 or more. This has a big effect in fuel consumption.

There's not an electrical reason, but instead a weight reason. By using the existing metal structure as a ground, it effectively reduces the number of wires by around half, and therefore saving a great deal of weight. (For example, otherwise each tail light would have to have two wires instead of one.)

Remember too, that some electrical loads in an automobile use a lot of current. A starter motor, for example, very commonly uses 0 AWG wire which weighs about 0.5 kg/m.

Interestingly, although not your question, the choice of negative versus positive is entirely arbitrary. In fact, back in the 1960s, Volkswagen used a 6V "positive ground" system for the Beetle up to around 1967 when they finally changed to the 12V negative ground system that is standard today.