The alternator HAS to generate A LOT of amps at a very wide range of rotation speeds. Further the electrical load can and will change drastically from moment to moment.
A fixed magnet dynamo would require an extremely beefy voltage regulation system to generate the required voltages and demands.
A much simpler, and in my mind, more elegent solution is the humble alternator.
Despite what people think, cars DO NOT have a voltage regulator. They actually have a "Field Regulator".
The alternator has a rotating rotor coil which generates the MAGNET part. This coil is made up of many turns of relatively thin wire. Around that rotor is the stator which contains the generation coils which are made of MUCH heavier wires capable of carrying the AMPS you need to operate your car.
In order to maintain a steady maximum voltage at the output of the alternator, the amount of electro-magnetism generated by the rotor coil is regulated by the field regulator. Since that coil has a high resistance, it does not use much current to excite it. In fact, when the engine is not started, it is excited from the battery through the charge light.
The beauty of this is that if you are not using any power the alternator produces very little load on the engine. When you need power, it is available almost instantly.
The parts are simple, cheap, reliable, and efficient.
BTW: Alternator is a bit of a misnomer. Your car alternator produces DC not AC, all be it with a significant ripple. All the above is contained within most alternators. Generator would have been a better name.
So why can't you do something similar with a dynamo? That is replace the permanent magnets with electro-magnets. Well, the truth is you could. However, you have to realize that a dynamo is simply an alternator with a mechanical rectifier. A dynamo requires a commutation ring and brushes to switch in and out the coils at the appropriate time in the voltage cycle.
That adds way more expense than a few diodes, has issues with wear and reliability, and is VERY electrically noisy. Further while commutating there are losses involved when coils are temporarily shorted by the brush or are left open loop and not giving you any power.
So if you were designing such a beast, you would think, hmmm.. how do I get rid of this mechanical rectifier... OOO.. Lets use these new fangled things called diodes... before you know it.. you are back at what we now call the alternator.
That is probably how the thing was invented in the first place.