We fought for loot, we fought because we liked fighting, and sometimes we fought simply to kick someone's ass and teach them who's boss.
Well, this has nothing to do with the estates, but to answer your comment:
Yes, there are plenty of reasons to fight, and I agree with all of them. If you want to have SMA atmosphere, however, you need to at least envision the possibility of winning. As you say, winning does not only mean expanding; however unless you RP your realm as a bunch of barbarians (which is fine, but not everyone should do it), then fighting for the sake of fighting does not work either.
If I take the example of Morek, we've fought for expansion, we've fought for defense, we've fought to impose our views on other realms, we've fought to shape continental policies, and we've fought just to show who's the boss.
And it worked. We don't need to fight to show who's the boss when our neighbors do everything we want them to do, don't we? Should we remove the ruler of a neighboring realm whom we like to have it replaced by one we don't like, only to start a war and put the old ruler back in place? No, of course, this goal is already achieved.
There are many reasons to fight wars, but expansion is the only one that is situation-independent. All the other reasons can exist, and do exist, and we act on them when they do, and we provoke them when they don't, but respecting SMA. But you shouldn't need to provoke a desire for expansion. It should be always there as a gnawing feeling in the back of your head: you are not strong enough, and if you were stronger you'd be better off. There should always be room to grow.