There have been a few card-based processors, but the cost to develop the architecture, design the modular components, and cope with the various possible iterations tends to keep the price on those higher than most folks like - it ends up being cheaper to buy something like Outlaw and sell it a few years later to help pay for a newer product than to buy into one of those card-based units and purchase updated cards. Plus you have to trust the manufacturer to come out with updated cards to buy.

The modding community is often very active in modding products like two-channel gear, but it never really has gotten into messing with receivers or surround processors - probably due in part to the sheer complexity of products like that. Analog side changes would typically mean surgery on printed circuit boards (yuck), power supplies in processors are generally designed to be plenty robust for the application, and the digital side is such a complex balancing act of logic that somebody outside the design team is going to have to spend far too much time trying to not break what's already there to make any improvements and still be able to make money off of selling those improvements. DVD players have developed a mod community, but I suspect that got started as region-free hacks and grew to include tweaks like power supply upgrades that wouldn't get too close to the heavy computational portions of the equipment.