@Zorbul : The gameplay answer is that Frontier have tried a "realistic"[1] approach before, and it turned out to be terrible. Case in point: most Notable Stellar Phenomena. With only a handful of exceptions, they appear only in small areas in the galaxy only, most of them being unique to said areas even when they are similar life, like the various small and large molluscs. Although the distribution of NSPs isn't the only thing to blame for why they turned out to be terrible and mostly unknown by the player base, despite their actual contents often being of better quality than Odyssey's static plants, but it's probably the biggest factor.
So, Frontier knew that if they'd try the same approach again(!), the same issues would most likely remain... but this time, it'd be in a paid expansion, not in a free update. So the player reception would have been much worse.
Now, gameplay should always have precedence above lore. (Assuming you want a game to play well and sell well that is.) So there's no lore in ED about why plants on thin atmospheric planets have either spread across the entire galaxy, or spread along specific galactic arms.
Although there
is one thing, from before Odyssey: that brain trees can be found around places where the Guardians used to be. So at least there was panspermia there.
[1]: let's not forget that we don't actually know how life is distributed in the universe, the only place where we have confirmed life is Earth. Who's to say what's realistic?