I saw you mention earlier that you tend to cap your population at very low numbers. This may be part of the issue, at least for those fortresses that have been capped at under 80. A population of 80 has been a magic number in the past in determining if you can be attacked by some of the larger threats like mounted sieges, so you might want to try running more than just one or two fortresses up past that mark and see if that breaks your dry spell.
Another possible cause of the problem could be related to that bug we had earlier in the release where the over world pathing was broken when determining neighbors. I remember for instance having forts that were literally right next to necromancer towers or goblin civs yet they would not show up as neighbors. I remember one forest fortress I had in particular where it should have had 4 or 5 necromancer towers in range yet not even half showed up as neighbors. Toady patched this bug which allowed the neighbors to show up properly again, but perhaps there is still some underlying over world movement and pathing problems that are still going on where civs that should be able to reach you still think they can't and thus don't send sieges? I assume you've checked to see if the neighboring hostiles have normal population levels? If they are dying or fledgling sites with only a handful of citizens they won't have troops to attack you with.
You mentioned that you always start your forts in a fresh world because you have very specific conditions that your embarks sites must meet. Perhaps it is something about the topography and other specific conditions you look for in an embark that is related to the problem. Have you tried just picking a completely random embark site with a lot of hostile neighbors to see if its the type of embarks you are choosing that is somehow related to the issue?
My final guess is that perhaps you are just astronomically unlucky in this regard and the game has been sending you mega beasts instead of sieges. I don't know if this still holds true in the new version or not since troops and their movements are now modeled in greater detail, but in past versions if you save scummed before a siege, sometimes instead of a siege a mega beast would show up and vice versa. It seemed like the game would determine if you met the conditions required to incur an attack and would then choose from a list of possible local threats and pick one randomly. To test this theory you could save scum repeatedly the next time you are attacked and see if it changes what you are attacked by or not. If this is the case you could probably increase the odds of being attacked by civs instead of mega beasts by decreasing beast numbers in world gen or simply letting world gen run longer. How long of a history do you run when you play? You aren't starting in like year 10 or something, are you?
The fact that others are reporting being sieged normally and you are not suggests it may be something you are doing different than everyone else that has uncovered the quirky game behavior.
Edit: I should add that in general sieges seem to be considerably less common in DF 2014 than in the older versions but they shouldn't be non-existent under normal circumstances. The decrease in sieges is a consequence of the fact that the game doesn't conjure the troops out of thin air like it used to. Anything that decreases the populations of aggressive civs will negatively impact the number and size of the sieges you can expect to receive. A couple of suspect world gen perimeters that come to mind include the number of mega beasts, history length and savagery. You might want to tinker with those variables if you have set them to unusual values during world gen.