Because Cache RAM is more Expensive than regular RAM, a hundred megabytes of the stuff will probably cost around $500. Plus, nowadays, Cache ram is optimized to be very closely linked to the CPU. This makes it so that if you made it big and off the CPU, it just wouldn't perform as fast or as stable. You would need motherboard circuitry that would be able to run as fast as the CPU to support the Cache speeds(it would be as if the motherboard was the CPU, get what I mean?), and no motherboard would be able to go that fast. And if you did have a motherboard chipset that went that fast, then Cache would become RAM, and there would be no Cache... get what I mean?
At one time cache ram was expandable.... the cache chips were on a RAM module and you could upgrade it like RAM modules. But that cache was so slow, modern RAM is now faster than that old school Cache stuff from back then.