Latest cell phone has quad core capability and faster CPU. Why is it that current fastest CPU router is only 680Mhz? Maybe, we should start using cellphone as router. Possible?
Don't see why not, but may not be best idea. Cells don't come with an Ethernet port. Some have USB ports, but I don't think it would really work well.
Cell processors are usually more specific to tasks that cell phones encounter. Number of cores and clock rate are meaning less and less these days.
Many people that want beefy routers will build their own using cheep computer parts. It would be cheaper than most cell phones, plus more manageable. There's a DD-WRT x86 build, and I imagine there's a x86 build for tomato as well. If you want, you could also try Untangle, pfSense, m0n0wall, SmoothWall, or IPCop. There are several others, but these are the popular ones with free options.
As for the why, most cells that sport quad core processors are very expensive without a 2 year contract. HTC One X's MSRP (I'm assuming this is yours) is $800 without contract. You can pick up a router that will handle all your home needs, and most small business needs for less than c-note. If I were to drop $800 on router hardware, it was have a much faster quad core, more RAM, more storage, and more options.