Keyboards and wifi are one thing (both already modular) but the CPU is integrated into the mainboard and an AMD-based board would presumably need different AMD chipsets and so on beyond simply the CPU itself. That's "modular" in the sense that the chassis is designed for swappable mainboards, but it's much more involved to support.

I remember socketed CPUs being a thing on some ThinkPad models.

I don't remember any laptop ever that could support both AMD and Intel CPUs. The chipsets are different.

Mid-90s had both laptops with CPU sockets, and pin-compatible drop-in AMD and Cyrix 486 and Pentium-equivalent chips.

More or less, yes. The different keyboard layouts is not that difficult to do. A different wifi chipset is trivial; customers can source their own from wherever they want. CPUs are a bit different, as Intel and AMD will require completely different mainboard designs. It's doable, certainly, but requires quite a bit of engineering effort, especially if you want to be able to place components and connectors (etc.) in the same places on the different boards, which is necessary for something like the Framework laptop, where they'd want to be able to allow you to put either mainboard in the same chassis.

Speaking of chassis, that makes offering different screen aspect ratios really hard, as you'll usually have a different sized/shaped chassis for a different aspect ratio. That might mean a different mainboard layout, different keyboard, different touchpad, and different battery, at least. That would vastly complicate Framework's offering, something I'm sure they're in no position to do as such a young company.

You can have your choice of 3 Intel CPUs, and you can bring your own NVME SSD, RAM, and wifi card. Of course the motherboard chipset determines which of those will work.

You can't change the CPU brand. Moving from Intel to AMD (or from the current Intel CPUs to a newer generation of Intel CPUs) would require an entirely different motherboard.

This ends up with a laptop where nothing really works well because everyone has a completely different setup and issues come up with particular configs. The reason the macbook works so well right now is every single part was very carefully supported and designed to work together.

I used to be all in on the FOSS hardware train but at some point you just want to get some work done rather than debugging wifi drivers.

You can only get so modular until you run into the fact that a hybrid Intel/AMD/etc motherboard chipset does not exist. Creating one would cost more money than they have raised.