Why fight over control of the few rare naturally habitable planets when you could build millions of Cylinders to live in?
If your civilization lacks the technology to leave the gravity well of it's Homeworld cheaply enough to establish the infrastructure that could be used to build Cylinders that's an acceptable answer.
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Magical fields are anchored by the mass of a fair-sized planet. You can use ships and stations for exploitation of resources, but spending too much time out of the mana well can cause soul damage.
What happens when you lose your soul?
You die, user, much like losing your blood or brain.
You could build permanently-habitable stations if you could sustain environment and defensive charms close enough to be within a star's mana well, but you're going to want a dense, cool star for that, Sol's not suitable.
But we also fight for the colonies! To rid them of the taint that is the Earth Federation and to move humanity to the next step of evolution and become Newtypes!
This is a far more classy option.
...
You turn into a motivationless husk of a person, going through the motions of life only out of habit. (This is great for turning prisoners into endlessly-working factory slaves.)
Because the space colonies are tremendously fragile, and living in them makes US fragile too.
Spin gravity isn't as good as the real thing. You will become physically weaker. Large portions of your population won't be able to survive walking on a real planet again.
Your artificial ecosystems are paper-thin, lacking the redundancy and biodiversity of a planet. It takes only one small disaster to cascade into the entire colony ceasing to support life.
That same lack of biodiversity likewise means weakened immune systems, because so much of the artificial habitat is sterile. This can mean that just traveling between stations in the same system can lead to epidemic disasters, a risk that spikes TREMENDOUSLY if anyone is traveling from a planet or even just a station in a far flung system that doesn't get as much contact with you.
Stations also have to deal with a critical lack of space to work with, which means you have to go with the most efficient forms of production. People on planets eat beef. People in space eat crickets ground up into a protein slurry that is 3d-printed into 'meat'. Eggs become something only the very wealthy can afford. You'll probably get fruit and veggies from the hydroponics, but not everything grows well in those conditions.
And, lastly, space stations are inherently predisposed to dictatorships. Because those in power can just turn off the life support of the rest of the station at any time, making uprising and resistance effectively impossible unless you manage to blitz them so fast they can't even vent your atmosphere.
Where do you think career politicians and "civil" service bureaucrats come from?