Because it's costly to maintain multiple discrete systems in a vehicle. If MCU3 can handle all the in-car duties as well as AP, they will do that from a cost standpoint. As said above, MCU2 is more than adequate for the moment, so we won't see an MCU3 any time in the near-term, just like we won't see AP3 any time in the near term.
I suspect AP3 and MCU3 will be part of the same package. The "upgrade" that AP2.5 people are getting won't be a true AP3, as I'm sure some hardware is going to be missing (SSLIDAR, among others) that will comprise the AP3 package. The current AP2.5 hardware is not adequate to handle autonomous driving in anything but pristine conditions. As we all know, most driving is not under pristine conditions. It will require handling virtually all edge cases through some sort of learning algorithm and sensors that aren't available on any Tesla at the current time. *IF* we see AP3 in the near-term, it will not be capable of autonomous driving, which is what everyone wants FSD to be, and it won't be that. We may have something called FSD, but it's not what people are envisioning. Not on current hardware.
Just a quick example... how do you differentiate a wheel bending/flat tire inducing pothole from a mark in the road or a shadow visually? Sometimes you can't. You need hardware that is not visually based for that. The car *WILL* have to handle that.