They are a fairly small company, and going for amd/intel means reaching the widest audience.
Linux on arm is very mature, but windows on arm not completely.
That being said, other companies could very well develop and sell boards for the frameworks laptop. So much so that iirc sifive did release a risc-v laptop board to use in the frameworks laptop case.
Linux on arm is actually pretty terrible outside of the server space due to their (Qualcomm, Imagination, and ARM) integrated GPUs being bad and having terrible drivers.
Is Windows on ARM still immature? I'd think with the Microsoft Surface (ARM processors for several years now) that Windows on ARM would be fine but I've never owned or used one so I don't have any anecdotal evidence, just my assumptions.
My brother is on the Windows side of the world (MS partner and all that, lots of CRM/DB work). He said if you stick to mainstream apps (MS apps, Adobe, etc) ARM is basically on parity with x86, tho not the performance choice. He said these days he rarely has customers come with support issues that boil down to "bug only occurs on ARM", though "it's slow on ARM" does come up. OTOH...If you have specialty/niche apps that you rely on, or apps from a small dev who doesn't have the resources to support 2 archs, your mileage may vary.
Linux on arm is very mature, but windows on arm not completely.
That being said, other companies could very well develop and sell boards for the frameworks laptop. So much so that iirc sifive did release a risc-v laptop board to use in the frameworks laptop case.