Why don't we have very high layer count PCBs (usually a maximum of 4-6 layers)?
Let's have a look at this iPhone PCB.



Notice there are no traces, only pads with devices stuffed right next to each other everywhere on both sides.

This is HDI (High Density Interconnect).

This is very neat. Basically you pay extra to have the 1-2 outer layers on one or both sides etched with extremely tiny features. The inner layers, which are mostly power and ground planes anyway, are etched using regular cheap processes.

Tiny microvias are laser-drilled into the pads to connect the surface to the next high density layer. There also blind and buried vias.

Simplifying things... the main problem with standard PCBs is vias. They go through the whole board and eat up space on all layers. You can add layers if you want, but they'll still be full of holes! And it gets expensive. You can't shrink a via hole below the size of the drill, and the drill has to be tough enough to actually... you know, drill the whole board without breaking... so it can't be too small. Also, everything has to align and register properly. Precision stuff is not cheap.

However, a microvia only goes through one or two very thin layers, so it can be drilled with a laser, and the hole can be much smaller. These, and also Blind/Buried vias free up space on other layers and allow to route more traces, and put components on both sides.

Each layer can do a lot more with these technologies.