I don't think that aspect is intentionally ignored, it's just that most FOSS projects are seriously understaffed, and the UX/UI part is not a high priority item compared to getting it to work.
I think that's a mistake, though. Users often become contributors, and by offering a substandard UX, you remain forever understaffed - you don't gain the momentum, because you're not attracting users.
UX should be a high-priority item.