A number of problems. A huge one is wiring. Parallel is really complicated electrically, noise drowns out your signal and things run slow. This is why most of o

A number of problems. A huge one is wiring. Parallel is really complicated electrically, noise drowns out your signal and things run slow. This is why most of our buses have switched over to serial (e.g. USB, PCIe, etc). Sometimes we run those serial busses in parallel, but that still works out to being easier. Timing is another huge one. Imagine running 2 million wires (for a 1080p display) that have to all be the exact same length to within some tolerance. The longer those wires gets the harder this gets. This is also another huge reason why the move to serial buses has happened. You can run 4 wires with really tight timings and the bits will fly, but if you try and run 16 wires all together, speed ends up dropping dramatically. Reality is that circuit boards don't have room for a large number of traces running in parallel of all the exact same length! RAM is a huge exception to this, but extreme measures have been taken to enable this to happen, a good chunk of your Mobo is taken up getting the RAM connected, and memory controllers moved onboard the CPU in part to get RAM closer to the CPU to simplify traces, Note this is all the perspective of a software guy who has to listen to the hardware team grumble for most of the day. :)