Uh they arent attacking enough make em more fighty! also any chance of making another race or just making gobbos more fighty also? what about beastmen? I NEED CONSTANT SIEGES- it would just be better to make some special thing you could make that ensures constant sieges and attacks so at a certain point you could just build it and wait for the fun.
Distance to their nearest civ definitely seems to affect their ability to attack you, so you might check where they show up in the embark screen. To help get some close by, you might try increasing the number of civs at worldgen, since it's unlikely that anything will kill them off (so more civs = more orc civs = more sieges). They definitely do attack, though, and are about as fighty as you can get.
In general, the more hostile civs you have, the more sieges you'll get. If you wanted more sieges, you could try making the Orcs more active (adding spring and winter to their active seasons) or creating more non-speaking civs with different active seasons (maybe a winter/spring, spring/summer, summer/fall (Orcs), and fall/winter civ?). I think you want to have many short civs rather than a few long civs so they refresh if you kill a siege off.
Once my current test fort is done (testing if I can get >80 orc sieges, and hopefully if they bring along animals/trolls as well), I'll see if I can think of some well-flavored other civs to throw in. The only problem I can see with constant sieges is that once they start, you'll have no caravans reaching you (I've heard sieges can replace caravans, and even if they didn't, it would likely die getting to your fort), and you'd have liaisons/diplomats dying left and right, which might mess things up a bit too much. As I said, though, I'll see.