There are really two reasons that cores haven't shot up. The increasing reliance on mobile devices means that power efficiency trumps raw processing grunt for most people, there's little point having the ability to process huge amounts on your laptop if doing so kills battery life.
cores can be shut down on a CPU until actually needed by the workload. And desktop systems do not have a power constraint. The hardware side of computing has a long history of providing processing power that many thought was not necessary.
The first desktop quad core processor was released in Nov, 2006