A chronograph can be done much more simply that any sort of perpetual. There is care needed in the design, but as the 7750 demonstrates, you can crank one out with no human interaction. The most important distinction is variable: Everything in a chronograph happens at the same rate all the time, A perpetual needs to handle the jump of the end of the month and properly advance 1/2/3/4 days, the right amount at the right time, no more and no less. This requires a care in not just design, but in manufacturing, that is not needed on a chronograph.
IWC implemented a rattapante on top of an existing chronograph in about 11 or so parts, no?
A