When has this ever stopped anyone?
I mean, your typical space empire usually takes a lot of inspiration from the age of sail, right? Where you had really close neighbours glaring at each other at home while their miltaries conquered shit all around the globe, often right next to someone else's shit, so why don't you get multi-planetary empires that nevertheless had to share planets>>50185462
Habitable planets (assuming they are habitable straight off the bat) are often pretty rare - in the history of the setting why wouldn't two or more space-civs see it as worth sending someone to once it's discovered? (think bongs, frogs and spics all sending people to mainland America)
At least once things get full space opera, because the first people there presumably come with a navy, but I could still see nations co-habiting a planet happening - even in space opera habitable planets tend to be rare.
IDK, maybe I'm just a bit bored with a whole planet being considered all as one thing