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.
Still bigger than a smartphone (it occupies the entire width of the laptop (when counting the heatsink, 2/3 of the width otherwise), and around one quarter of the whole area).
Also, your original sentence was
> Modern laptop motherboards can easily be about the size of a cell phone.
Here we see that the one and only best laptop in this regard comes close to the cell-phone-sized motherboard. That's far from what “modern laptop can easily be” means. Most laptops sold today have a much bigger motherboard than this.
(BTW, as much as I'd like to see non x86 laptops going mainstream, software compatibility is huge issue for an ecosystem as diverse as the Windows one (and Roseta 2 being too much of a coinflip isn't encouraging…), we might get there at some point, put it's gonna be slow and painful.)