The car components are made to different spec than consumer electronics (for starters, there are several types of component, depending on what "range of temperatures" you need to operate them in; e.g. the stuff that goes into the traffic lights in northern parts of Europe is not something you buy off Farnell for your hobby project), and they also need to be reliable and tested. Add, of course, the usual graft and overhead.
Another thing is that a engine controller doesn't need a near-GHz quad-core chip and shit ton of RAM. It's not meant to stream HD videos or run Python, it has to run a bunch of feedback loops fast enough. Less powerful chips tend to be cheaper and more reliable.
