Please get a grip: ARM SoC's are not full custom hardware....
They are unique, yes, but you get a standard processor, a standard bus system (AMBA, the ARM on chip answer to PCIe), a recognised programming model with very good compilers, hardware debuggers, etc. You also get access to a huge library of AMBA modules rather like PCI peripherals from 3rd parties. It is just like assembling a custom PC but much harder.
You get TSMC, Charter, STMicro or one of the other foundries to make them for you cheaply. Lots of CMOS bulk manufacturers out there, only Intel and IBM have their own CMOS foundries now.
What you get is the huge power advantages of having it on one chip (power savings for no pad ring, no unnecessary extra hardware and no pointless charging of PCB traces). You do need to engineer it for low power but they have a team hugely experienced in successful designs - PA Semi.
You also get ultimate lockin - no 3rd party OSs and noone else can use your software.
Lots of programmers in the embedded space who know what an ARM looks like.
Google make their own ethernet switches and PCs for the same reason - done carefully and smart it can be of huge value..
Apples PPC processor was a fail because they relied on Motorola and IBM to design it. Motorola gave up the PPC and IBM doesn't do mobile - only big iron. Apple didn't want to do a desktop and laptop processor and chipset. It takes Intel teams of 100s to design them. ARM SoCs are much easier.