Owners can't always be trusted to follow the recommended service schedule.
The service now indicator light is an attempt to help push owners to get the vehicle properly serviced. Also, the conditions that have this service indicator light on are variables and can differ from owner to owner. Thus the car is flagged for service not so much on a fixed schedule but as usage, climate, etc. dictates.
Porsche has a lot of risk -- via potential warranty claims -- with new vehicles or CPO vehicles so it makes sense for Porsche to do what it can to ensure that every warranty claim is legitimate and not due to lack of proper servicing by the owner. At some point improper servicing can be interpreted as abuse/misuse of the vehicle and the warranty is not for addressing problems arising from this usage/treatment.
This service now indicator and the requirement it be dismissed by a Porsche diagnostics computer is another way Porsche has of tracking that a car has or has not had proper servicing. I've heard/been told directly that some Porsche dealers will reset this service indicator but charge for it. One reason is it requires the use of the Porsche diagnostics computer which is very expensive to buy and operate. ($600/month is the number I recall.) And because of this most dealers just have the one. And the reset is not just a snap of the fingers quick operation either. The computer must scan the car's entire array/population of controllers and the process takes minutes. Used to be when I visited a dealer service department the diagnostics computer was shoved off in a corner. Nowadays it is almost always beside a vehicle connected, actively being used.
Also, the dealer and the factory too, as the diagnostic computer is in contact with the factory whenever the computer is in use, has a record the car's service indicator now was cleared without the service that was due being performed. This builds a record that can be used to disallow warranty claims should the car come in for one. Later in the vehicle's life, this info could be used to filter out trade ins and lease returns as they come in due to lack of documented proper servicing.