This is not due to the fact that faster RAM could not be built, as explained in the previous section. It is possible but it is not economical. RAM as fast as current CPU cores is orders of magnitude more expensive than any dynamic RAM.
Actually it's not true anymore. While it's certainly true that it's possible to build RAM cells which are as fast as CPU cores it does not mean that it's possible to build RAM subsystem which is just as fast. And in fact it's totally impossible today. Not for $1'000'000, not for $1'000'000'000, not even for $1'000'000'000'000. Why? It's easy. Speed of light is 300'000km/sec (exactly 299'792'458m/s since October 21, 1983). It's not uncommon to have 3GHz CPUs today. This means that information can only travel 10cm in one tick - and it should travel back and forward, so only 5cm - less than 2in! If you'll take a look on DIMM you'll find that it's length is 133m. More than 10cm! So even if you plug DIMM module directly in CPU and cells on said model of RAM have infinite speed - you still can not get RAM which is as fast as current CPU core!
Sure - we can make geometrically smaller module and put it closer to CPU, but if we'll want to have gigabytes of RAM - we'll be forced to store only parts of this information in this superfast RAM, the rest will go farther from core... oops - we just invented the cache again! So no, this is not just question of the economics anymore - IT industry stumbled upon first fundamental limitation few years back...
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