There is a massive difference in terms of design methodology.
A PC motherboard has oodles of space and really doesn't have to be concerned much with physical envelope design.
Mobile phone board design is the complete opposite.
The higher density construction of mobile phone boards would also make them more fiddly and expensive to assemble and test.
Dave.
My IBM friend is going haywire because he's feeling under-appreciated.
Computer motherboards regularly have 4x to 20x more traces than a smartphone while having to deal with higher frequencies over larger areas.
Multiple power systems, long lists of standards to comply with, lots of ICs to engineer in and sockets to cause trouble.
The smallest mainstream motherboard standard, ITX, generally has 3000 traces.
ARM smartphone... BGA-423 + other stuff, so 1000 traces, space is tight which makes design hard, but you don't need to worry about RF wizardry since the only HF thing is the RAM chip located 1 cm away, not several inches, and generally 200MHz, not 600-900 like modern desktop RAM.
Phones are a bitch to route though.
Laptop motherboards, those are harder than desktop or phone...
Servers can be brutal... 2-8 cpu sockets with 1366 pins, 4 to 32 240-pin memory slots, and IO slots, and lots of integrated stuff.