Reply #1:
Why? Because how else do you expand the universe beyond what it already is? Of course, do you just keep adding more of the same thing, or do you spread it out among the galaxies? …the universes? …the dimensions?
Question…
What could be so ambitious, complex and/or otherwise time-consuming, or what could be so important to the very heart and soul of NMS, than to provide any real reason to travel beyond a single galaxy? …spread it out.
Answer…
What could be more far-reaching than the ability to travel beyond all of them, and into a multiverse of parallel universes and underworld mirrorverses, except only for a reason to do so? …spread it out.
Licks finger to the wind: 
"Ah, yes. Coming soon!"
Reply #2:
Sean Murray said: "For a while now we’ve been working on something pretty ambitious in the background. It’s a small team but we like it that way.
“Similar to No Man’s Sky, it’s the kind of project that even if we had a thousand people working on it, it’d still seem impossible.”
The game was in “very early” development as of September last year but Murray reassured that, while No Man’s Sky is still receiving regular updates and isn’t finished by a long shot, the two games aren’t impacting each other’s development.
“No Man’s Sky is not being held back by the next project but neither is the new thing being slowed by No Man’s Sky," he said. "We’re lucky enough to be able to allow folks to move freely to work on what excites them.”
Quoting Sean…
NMS isn’t finished receiving updates by a long shot. HG moves freely between both games, and equally.