Also, regarding all the people jacking off to "well you can just RKV/asteroid strike a planet so nothing else matters", 1. y u no fun, 2. there's no reason to actually do that, because you usually lose more that way than gain. Unless you are or you face some super xenocidal species, but that's an edge case.
This is already super off topic but I wanted to point out something I think Sci-fy doesn't consider enough:
When it comes to different species fighting, it's probably safe to assume that the environment one lives on is not-easy-compatible with the other's biome; it doesn't have to be "they breathe methane we breathe oxygen" it could be more like, "their homeworld has 1.32Gs and half as much oxygen and constant 40% humidly and our food is toxic to them and ect. . ." And the crustal layers of planets are not as heavy-mineral rich or as easy to mine as a good quality asteroid field. So the gain via taking over an alien world may not be all that incentivizing.
That said I do agree, why be aggressive with an alien civilization in the first place?
Space is big and full of resources, and Information/data/knowledge is valuable trade, so why give yourself the crippling anxiety of RKVs/asteroid strikes/black space paint/biowar weapons when you can just like, not escalate it that far?
Find/replace 'city' for 'planet' and you'll realize exactly why this logic would predominate.
Yeah but that goes both ways:
Why blow up a city when you can just take out the military base?
Why kill the planet when you can take out the space-docks?
Plus you and your enemy would most likely have your space military's infrastructure off-world and at least a short ways away from your home planets, if just for logistical reasons and nothing else. DeltaV always has the final say.
I would think if you wanted to stop someones space war-machine, it would make much more sense to deny the enemy SPACE then planets.
To make one more analogy: If space is an ocean, then Planets are NOT the islands, they are the supporting country's/continent's inland government, all the naval/space infrastructure in on the coast/in orbit, and the islands are space stations/dwarf-planets/mining sites in-between.
TLDR: Most alien's planets are probably toxic to each other and a space navy is gonna get all it's ship parts from factories that are in space too so attacking planets might be kinda unnecessary.