Probably for the same reason that a skeleton can still taste food and drink even as it doesn't need it, can smell without a nose, can touch without skin or nerves, and hear without the apparatus necessary to convert sound into information.
If you really want to walk the logic of "they don't have eyes so they should have darkvision" to it's logical conclusion, I think you'll find most narrative sensory experience totally useless for skeletons. It's still a game, not a perfect simulation.
Yeah, it's a little weird how Skeletons are the only Undead option to lack a feat to allow them to get the advanced undead benefits. Maybe it'd be the must-pick for their ancestry feats VS other undead's class feats? Maybe it's the fact other undead can pick the feat starting at level 6, but skeletons only get feats at levels 1, 5, 9, 13 and 17? 5th would be too soon, but 9th would be too late?
I think a simple solution for Skeletons would be an addition to their Undead section on the right-hand Mechanics side. Add that at 6th level, you gain the advanced undead benefits.