it's a mix of both: designers and coders who figure they know best what's best for everyone. but as most things in life, even ergonomics and accessibility, there's no one size that fits all perfectly, and then, those of us who know what's best for ourselves, or wish to experiment to find out, may be somewhat deprived of that opportunity by others who think they know what's best for us.
it's been decades that it has seemingly embraced the "the less choice the better", in stark contrast with earlier traditions of flexibility and user-driven configurability in free software. there's something to be said about managing complexity there, but I suspect there's also a significant element of corporate influence behind this