Seems like a deliberate design choice, not some inevitable consequence of modernity. Why exactly does the board need to be cell phone sized when the actual form factor of even a netbook is substantially larger?
Given the option between thicker w/ a bit more battery life or thinner and lighter, consumers are choosing thinner and lighter.
> ...consumers are choosing thinner and lighter.
A) I'm guessing actually lighter more than thinner.
B) Are consumers really clamouring all that intensely for "Thinner! Thinner!", or is that a) because that's all that's offered, and b) the relentless hype from manufacturers -- above all, bendy phone maker Apple -- amplified by rarely-critical media?
For instance, in a iPhone 11, the battery occupies less than half of the surface of the phone[1]. On a Macbook pro, it's about the same.[2] [1] see this ifixit video if you want to see it by yourself: https://www.youtube.com/watch?v=Feyqwf3cYtw&t=161s
[2] https://guide-images.cdn.ifixit.com/igi/noPvQ2CLs1jptX2p.hug... You can see that the motherboard is much, much bigger than a smartphone.
It doesn't matter that the rest of the form factor is larger: there's not a lot of empty space in these, so a larger main board will still mean a larger and heavier device.