So you're saying that Seldon's Plan, while well intentioned (reducing the duration of a Galactic dark age) was still an attempt to control the subjects of an Empire? Intriguing. ;-)
Asimov rather famously related the rationale for this. Publisher Joe Campbell required that humans always be shown as superior to aliens. Asimov thought this was silly, and simply left aliens out.
One of the non-Asimov Foundation books -
Foundation's Triumph, by David Brin - provides an explanation for this building on
Blind Alley, an obscure short story by Asimov. And that's understating it -
Triumph is a tour de force of rationalizing all of the unbelievable aspects of Asimov's future history. Brin is brilliant at it, not just taking on easily noted tropes like "humans only" but pointing out many more gaping holes, and coming up with ways to explain them that give the characters involved much needed depth. (The other two "Killer B's" Foundation books are decent reads but don't advance the meta-plot of the series in the same way as Asimov's later entries (
Prelude and
Forward) or Brin's
Triumph).
FWIW, my preferred reading order would vaguely match the published order:
Foundation (Foundation era)
Foundation and Empire (Foundation era)
Second Foundation (Foundation era)
The Stars, Like Dust (Galactic Empire era)
Currents of Space (Galactic Empire era)
Pebble in the Sky (Galactic Empire era)
Blind Alley (Galactic Empire era, short story)
I, Robot (near future short stories, with bridging narrative)
The Caves of Steel (Robot/Spacer era)
The Naked Sun (Robot/Spacer era)
Mirror Image (Robot/Spacer era, short story)
The Robots of Dawn (Robot/Spacer era)
Foundation's Edge (Foundation era)
Robots and Empire (Robot/Spacer era)
Foundation and Earth (Foundation era)
Foundation's Friends (various authors, short story collection)
Prelude to Foundation (Foundation era)
Forward the Foundation (Foundation era)
Foundation and Chaos (Gregory Benford, Foundation era)
Foundation's Fear (Greg Bear, Foundation era)
Caliban (Roger MacBride Allen, Robot/Spacer era)
Inferno (Roger MacBride Allen, Robot/Spacer era)
Utopia (Roger MacBride Allen, Robot/Spacer era)
Mirage (Mark W. Tiedemann, Robot/Spacer era)
Chimera (Mark W. Tiedemann, Robot/Spacer era)
Aurora (Mark W. Tiedemann, Robot/Spacer era)
Foundation's Triumph (David Brin, Foundation era), also Denouement
A purely chronological reading won't give you the meta-plot that emerges through the series. A relatively spoiler-free timeline is at:
http://www.asimovonline.com/oldsite/insane_list.html - note the huge number of Robot short stories; while these are all fun, the selection present in
I, Robot with the bridging narrative is sufficient. Also note that most of these works were never intended to be part of a single cohesive future history, so you can't read them too closely and expect continuity.