When Samsung and Apple are seemingly incapable of producing repairable devices while Fairphone comes out of nowhere and are doing it just fine, it's not a question of it being difficult. It's a question of it being profitable. If Fairphone managed to make a repairable device, Apple or Samsung would have managed to create one that is both fully waterproof and repairable if they considered it a priority.

From the view of the industry, the logic is that "if you're not going to have your phone break down in heavy rain, at least make sure you'll have to replace it in three years because the battery starts sucking". There's absolutely no incentive to make a product that will actually last.