It's all about the function such clothing serves in the context of the show. Your image is a perfect example: that fine gentleman is a member of the great Centauri Republic, a grand old empire where it's not just the clothes that are reminiscent of a proud empire at its peak (and approaching its decline) - their jewelry, customs, buildings, spacecraft, and entire social and political structures follow the same design principles. But even if I didn't know any of this beforehand, that's the impression I'd get just from looking at this image in any event.
The reason for this, in fantasy and science fiction, is visual shorthand: in other words, the audience can very quickly pick up a lot of visual cues about the type of alien society being shown, without the writers spending ages having to explicitly outline it all.
Also note that in this case, such clothing can't be considered "anachronistic", as it's entirely correct for its context.
However, to directly address your questions:
Is it reasonable for spacefaring humans to wear clothing similar to that of the 1700s - or even older apparel (such as a toga)?
You're in a starship, or on a space station. Who cares what you wear? Would a Starfleet jumpsuit or Jedi robes give you more protection from hard vacuum than Centauri court dress? Of course not. It's an indicator - both in and out of universe - of class, position, status, and possibly character.
Note that in Babylon 5, the show this image comes from, the Starfury fighter pilots do wear spacesuits while flying. Appropriateness in context.
What could be a reason for people wearing old clothing styles in such a futuristic setting?
Coming back to the discussion about what's retro and what's just old, it's not necessarily either of these things. It's symbolic, and it's evocative. Out of universe, it gives the viewer a summarised version of what to expect of the wearer. In-universe, it's a symbol of society or beliefs.