Why is it that Military Posers all use the same story - Ranger, Seal, SF, etc.?

You're on target "Top"....

Because us Signal Corps types take imposters more seriously?
:-)

Too many WANNABES

Well if Hollywood hadn’t made a bunch of movies with high speed Soldiers and Marines (and others), in SF, and John Wayne Green Beret etc....then would there be all these poser’s? Think about it.....

1% of Americans join the military. Of that .25% go on to special forces. The news isn't going to report what the Machinst Mate did in the Navy or what the Cook did in the Army or the Supply Clerk in the Marines. So unless there is a military background in that family the only military these people know is what the news reports. So it's easy for them to look up Seals, Rangers, Green Berets, Delta Force and Divers. After all even Hollywood knows all about them. Why do research on the rest 75% of the 1% elite.

I think your explanation is pretty good as it is. The only thing I would add is that these morons are usually Walter Mitty types who play out their fantasies in a uniform loaded with ribbons, badges and medals that they play with in their closets... then they dress up and play soldier. You know... some people play with matches in the closet... these idiots play with medals and ribbons they buy at the surplus store.
One of the most interesting cases I ran into was a guy who had actually served and apparently had reached the rank of Major. However, to hear him tell his story, he had graduated from West Point, he'd been a SEAL (although he had an Army ID) AND he had retired as a Major General (no less). The universe of Major generals is not big, with most of them serving as state TAGs, so perhaps that wasn't a good choice of "ranK". In any case, a quick check eliminated the "general officer" option. Another check with West Point's AOG zapped the USMA graduation story. So I just decided to laugh about the SEAL portion of this person's "experience".
I guess that the best way to think about people like this is to think of them as pathetic clowns. As someone else mentioned, most veterans don't go about in full dress regalia with ribbons, medals and other miscellaneous badges and what-nots dangling from their chests. There is no need. These impersonators are just poor, insecure fools that try to bolster their miserable self-image by pretending they are something they are not... and hoping that they get "admired" for their fake representation. It's ridiculous.

In my opinion they always want to be what they could never be

What I see on the guy on the right is the fact he is not even old enough for one of the ribbons he is wearing. The RVN PUC was only awarded in the mid 50s - 1954 specifically, for Indochina advisors. From Wiki-

U.S. recipients[edit]
State of Vietnam Friendship Ribbon:
Military Assistance Advisory Group, Indochina[8]
Citation: During the months of August and September 1954, the members of the Military Assistance Advisory Group assisted to freedom from the terrors of Communist rule in their own territory of North and Central Vietnam, hundreds of thousands of men, women, and children. Thanks to the efficient and humanitarian assistance of the members of Military Assistance Advisory Group these refugees have been given an opportunity to start their lives anew in the free territories of Viet-Nam. The free people of Viet-Nam express their heartfelt gratitude for this unselfish manifestation of friendship and support.[4]
Task Force 90[9][10]

Delusions of grandeur.

Because no-one wants to be a fake 88M Specialist who failed his PT Test and shot marksman... People want to both emulate and impersonate the best!