the infiniti/
armada intellikey (IK) system is pretty tied into the car. you can set the fob to control not only the seats, mirrors, steering wheel, but also the HVAC controls, radio settings specific to the fob.
still, nissan could have added into the system the ability to allow autostart, however the cost may have been beyond the scope of the cost they were trying to meet.
they also probably figured more people would not NEED autostart vs. the ones who say its a deal breaker if they dont have autostart. and the ones who are the deal breakers usually post up on site asking about why company X didint install _____ (insert feature).
you can install autostart and in fact the nissan IK system can use the FOB to initiate autostart. the MAJOR con is you need to be within IK range for this to happen (within 3 feet).
right now, on the nissan system, if you have an aftermarket autostart/remote start installed, you have to get in the car, shut off the car and then restart it with the FOB you have with you. again this is because when the car autostarts remotely, the gadget (i forget the name of it at this writing--its installed to make the remote system work) fools the system to allow it to start. once you get in the car, the system needs to ID the fob thats on you, and by doing this you need to turn off the car and re start.
every car has some features other cars dont. when shopping for a vehicle, pick the car thaat has exactly what you need or has a fair compromise.