Why do Apple's 2012 MacBook Pros cap the RAM limit at 8GB?
The limitation imposed is based on the chipsets used on the computer's motherboard. For example, the noted Apple maximums you mentioned for the MacBook Pro and the Mac Pro, are smaller than the maximums that those machines can address in practice. The MacBook Pro tested maximum is 16 GB according to EveryMac's MacBook Pro max RAM listing. And as for the Mac Pro, its maximum is 128 GB according to EveryMac's Mac Pro max RAM listing.

As noted on Intel's Specs, the CPU supports a different maximum than the chipset on the motherboard does. The Xeon X5670 used in the Mac Pro supports up to 288 GB.

And as for the i7s e.g. a i7-2860QM used in the MacBook Pros, it supports up to 32 GB.

So these limitations come from engineering decisions made by Apple based on what kinds of chipsets are selected to be installed on the motherboard and what those chips support is what enforces the maximum amount of RAM that a particular Mac can address at or below what the CPU can actually address.

As for why Apple underrates their maximum numbers for RAM on some Macs, that's for Apple to know and for us to wonder. However, admittedly it is a nice practice to under promise and over deliver for whatever reason Apple has. Although, the Mac Pro supports more RAM than Mac OS X can address, according to OWC's testing on Mac Pros, where they discovered Mac OS X unofficially will not address more than 96GB of RAM, but other 64-bit operating system can get to the full 128GB.