I don't know which boards you're looking at, but high layer counts are definitely used where it makes economic sense. Have you looked at the motherboard of a PC or cellphone lately? I regularly work on compact special-purpose products that have anywhere from 6 to 12 layer PCBs. In particular, high pin count BGA packages require a certain number of layers just to make the connections (a.k.a. "fanout") to the inner balls.
But part of your question doesn't make sense. You can't in general replace a 10 sq. in. board that has four layers with a 5 sq. in. board that has 8 layers — it doesn't work like that. Remember, components can only be mounted on the outer two layers, which puts a lower limit on the area of the PCB. Connections between those components and the inner-layer wiring require vias that also take up area on the outer layers. Blind and buried vias can somewhat mitigate the amount of area required for wiring, but they add additional processing steps and cost to the board, too.
In many cases, the size of the board is dictated less by the number of components and more by the placement of external connectors, etc. that makes the most sense from a packaging (and user experience) point of view. For example, using a single "oversize" PCB that stretches all the way from the front to the rear of the box may make sense if it eliminates the expense of making two separate assemblies with cabling between them. Then the designer has the "luxury" of spreading the components out a bit and using fewer layers. The final BOM cost is often lowest using this approach.
Responding to your edit about IC design: Actually, ICs have only ONE layer of active components, which is even more restrictive than a 2-sided PCB. However, the minimum feature size of the active layer is typically much smaller than that of the metal wiring layers above, so there's considerable benefit to having multiple wiring layers.
The limiting factor becomes the fact that the vias from any wiring layer to the active layer must go through all of the lower wiring layers, limiting how much wiring can actually be done on those lower layers. Therefore, the lowest layers tend to be used for the "most local" connections only, and the higher layers for the more far-reaching connections and global connections such as power supplies and clock signals.