I’ve done more dev work than design, but I’m educated as a designer and do both professionally. A big part of good design practice is studying what your users do and making decisions based in that. Assuming your use case is representative enough to reason about your users’ needs and that interface design is unnecessary with a solid enough UI kit is exactly why designers need to be involved. It’s also why almost all FOSS UIs are garbage.

Most users don’t use the same app on many devices and operating systems. That you consider the OS a constantly changing fungible component in your daily tool chain is incredibly unusual. Outside of some dev and IT tools, optimizing for that use case just doesn’t make sense and even most IDEs don’t try. Most device UI kits are more carefully designed to suit their users interaction needs than any app could hope to achieve, especially considering accessibility.