There's no reason they couldn't use smaller capacity modules. SOCAMM has better area efficiency and z height, both highly relevant for thin and light laptops.
The form factor is only a tiny part of the RAM module price. DRAM already isn't compatible gen to gen so you might as well go with the most optimized form factor
I agree, competition and economies of scale are much larger factors. Custom RAM SKUs for a single manufacturer with no competition will obviously cost more than an industry standard part. And you'll be locked to buying RAM upgrades from framework themselves.