Nomenclature detail: strictly, "generators" make DC and "alternators" make AC.
A motor-generator set has a motor driving a "generator", which strictly means a DC generator.
That's not true. You have plenty of diesel-powered generator sets that generate AC:
https://www.survivalcenter.com/Diesel%20Generators.htmlI would even argue that AC generators are way more common because they are simpler and more useful (most appliances meant to run from mains need AC) than DC which would require an inverter.
Or even electrically powered sets:
https://www.horlick.com/products/motor-generators/A rotary converter has an AC motor driving an "alternator", which generates AC. These are commonly used to change frequency, as in the 400 Hz case above and electrified railways (e.g., the Swiss railways that use 16-2/3 Hz AC, a subharmonic of the 50 Hz commercial mains).
That's not quite true either or rather it is not the only type. Read the articles I have posted and watch the video. They explicitly talk about rotary converters with DC output. That's from New York subway which was converting 25Hz ("cycle") AC to 625V
DC used for the third rail.
These actually predate the use of the frequency converting types by a lot - the substation with the converter from New York was built and operating roughly 10 years before Switzerland has even adopted that 16 2/3Hz for railways and 2 years before the first line electrified by this type of system (at 16Hz) by Siemens started to operate in Bavaria.
Also, rotary converter is
not a "motor driving a generator" (i.e. implying two machines connected by some shaft, belt or gears), even though the function principle is the same. The two machines actually share the same armature and field coils, making the whole assembly smaller, lighter and more efficient than having two units connected only mechanically, as is common in motor-generators.