Why is it impossible to find certain watches?

The Flighty is quite high in demand. For these high in demand Seikos, I've had better experience in online shopping rather than going to brick and mortar stores.

I ask kinda The same question to my AD on how come he only carry certain brands he mentioned it’s first it’s expensive also it’s exclusivity! he mentioned to me he cannot carry GS even though he’s a Seiko dealer since the pre requisite is that he must also sell Rolex or there should be a Rolex Dealer close by!! that’s insane GS! also You have to purchase the inventory as well so that’s tough especially if your a small boutique. So for small stores like my AD he prefers ordering what you wanted and knowing there‘s a buyer for it instead of stocking something that light or might not sell, but he does carry certain things off course but not all.

Most ADs are part of a larger retail operation, and those larger retailers have "buyers" who do a bunch of analysis on what's selling, what's not, where, etc., etc., and then they distribute inventory accordingly.  

The good news is that if you don't see the particular watch you're interested in, you can ask folks there to check the system to find out what other outlet of theirs might have that particular watch in stock to try on.

I have zero experience with ADs, but I spend a lot of time observing the watch world.  I've gleaned the impression that there is a degree of gatekeeping when it comes to the most in-demand watches, to the extent that you can't simply walk into a Rolex outfit and expect to get, say, a Submariner even if you're fully ready to pay for it.  

Like most things, unfortunately, it sounds like there's a game to be played where you need to pay your dues and establish a 'relationship' with the AD by buying less 'in-demand' offerings that they are looking to move until you've got seniority or something.  That, or be a person of relative importance, like a celebrity or pro athlete or politician.  

I mean, I can't say this is the story across the board by any means, and I can't attest from experience, but I've just heard this story so many times, it seems to be the norm.  

I suspect the Flightmaster is much like the a few of the other heavily hyped Seiko models, in that it's only popular inside the watch community. Most ADs only want to stock what they can sell to any muggle off the street, unless it's a rare, high margin watch. 

Your best bet for oddball watches is online, as some online ADs specialize in serving the watch community. 

MegaBob nails it.
Using Rolex as an example; There is a lot to deal with before any watch hits a customer's wrist.  If the store is part of a larger chain, there is usually one buyer (or one regional buyer) who is responsible for ordering. 

1. They buy based on a number of units, but they do not get to necessarily say which models. For rounding purposes, say they buy 100 watches for an allocation at $10K each, total spend of $1 million. That is not a small spend for any company, regardless of size. 
2. They then need to figure out how to distribute those 100 watches to their 10 stores. That breakdown may result in 5 Datejusts (of varying sizes and materials), 2 OPs, 1 Sub, 1 Yachtmaster, and 1 Cellini going to one store, and perhaps a top-performing store will get a more favorable breakdown that may include a GMT or Daytona.  Regardless, the AD had to purchase a lot of undesirable watches that they have to unload to get a few hot pieces. As Megabob pointed out, that is a lot of money to lock up on hopeful sales. At least with Rolex they are easier to unload. IWC, GS, etc?  Tougher proposition, which is why they may not carry a lot.
3. Once the individual store gets their allocation, they then allocate to SA. Top performing SAs get a better allocation. On average a single SA may get one or two Pepsis a year. Daytona? Ditto. Those top SAs tend to have a book of clients that buy with no hesitation, so the average person off the street may not always get to interact with the best SA, thus not getting to deal with someone who has a good allocation.  This may not be the case at all ADs, some may equitably allocate, but your mileage may vary. Again, this is more specific to Rolex and other hot brands. 

As far as Seiko goes, I imagine there are a couple of factors:

• Most Seiko ADs likely do not care about allocation. A place like Macy's just wants pieces to fill the cabinet. Their average shopper is not a watch enthusiast.
• Others are jewelry stores that similar to Macy's want inexpensive alternatives, who again, do not cater to enthusiasts.
• A newer, perceived desirable model may only be distributed at allocation breakpoints. A dealer may not find it worth spending the extra amount for a couple pieces.

Thanks everyone I've got a much better grasp now thank you for commenting