Is there a reason Allied Seal gear is still so incredibly expensive?

[Reinha]

For a less exaggerated figure of it's cost, simply do the math. It comes out to a total cost of 5,540 Allied Seals for a single job's set, which takes 11 weeks (3 months) to farm up if you do every applicable hunt for the duration.

You can sort of grind A and S rank hunts as well to make it take a bit less time, but that's pretty much impossible to really math out due to the many organized linkshells clearing them out on the timers.

http://ffxiv.ariyala.com/HuntTracker/siren

There's the tracker I use for hunting. A ranks spawn without conditions and most S rank conditions are easy to do on your own. You don't need to wait for a LS to find hunts for you.

I don't know how you figured it would take 11 weeks to get that many seals. That's only 500 per week. There are 17 A ranks in ARR, each giving 40 seals and they spawn every 4 hours of so. Kill 5 per day and you have 1400 seals/week just from that, not counting S ranks.