E.g sockets and chipsets change and will force incompatible changes, no matter how much framework would like to keep things stable.
Framework does work with ODMs (Compal, I believe, is their main one?) to design mainboards for their chassis, which are designed specifically for Framework. It's not like they just take an off-the-shelf design and build it without any modifications.
And yes, chipsets change. (A "socket" changing isn't really a thing when we're talking about a laptop where the CPU/SoC is soldered in.) Generally this isn't a problem, though: as long as you can design something that physically fits in the chassis and supports the features you want, you're fine.
I believe the framework CEO himself mentioned in an interview how the chipset and socket are kinda at the core of designing the whole laptop, because it necessitates the placement of the cooling and all other components. I sadly didn't bookmark that YouTube video, so I cannot provide a link however
And fwiw, Apple is the only company that could make their laptops fully compatible and upgradeable, because they've got the relevant stack under their own control. Sadly, they're not interested in reducing ewaste, as that would mean less profit for them