Can someone explain war attrition to me?

[Paradoxical]

So I'm in a defensive war. I figured this race would be highly aggressive and would declare on me at some point so I built a fully kitted out star fortress to defend their only entry point to my empire.

So it's been a three years since they declared. I have 22% attrition and they have 15%. So the AI realized that it couldn't take down my starbase and has done nothing but they are "winning" the defensive war.

Why is this a thing. The war has no events, they haven't even attempted to take their claims. Do I have fly a fleet over there to strike at them even though I'm in a defensive war? Is this a tech difference and can they take the claims without firing a shot?

So weird.

[TheMac]

Welcome to Stellaris. Sounds like you're someone who recently discovered the franchise. The war system works on a principle that your people and their people will get tired of the war at some point and call it quits either with a "white peace" where they settle the status quo (whatever they own they keep and what you own you keep, depending on claims and races, etc.) or some type of conquest where they wipe you out or you wipe them out.

In a defensive war, you can either let them build up their fleets and keep bashing their fleets against your star bases or you can go on the offensive. If you kill their ships, their people become demoralized and their war exhaustion increases, though for every ship they kill or every system or yours they take, your war exhaustion increases. War exhaustion also increases simply over time, because people get tired of the shortages and all that goes with war. Here's the wiki link.
https://stellaris.paradoxwikis.com/Warfare#War_exhaustion

Your choices are to wait it out and keep checking to see if they want to declare peace or go on the offensive and raise their war exhaustion through conquest. Best of luck!

[Half Phased]

The attrition you’re seeing is a mechanic to stop wars continuing forever if the defensive bastions are too strong to allow invasions. It’s there mechanically as a timer.

The reason why your opponent is gaining attrition slower than you is because they have techs or other buffs that lower the rate of war exhaustion gain, which applies to everything, including attrition.

But it doesn’t mean they’re winning the war, it just means that your empire is closer to wanting to give up and stop fighting.

[mortache]

Do you have more powerful fleet or stabases than their fleet? If that is the case the AI will not attack on its own.

[Ryika]

Attrition does not mean you're losing the war. It just means that 2 years after you've hit 100%, the opponent can force a status quo and end the war.

If they're not occupying any of your territory, they'll not get anything out of ending the war this way.