I mean, except they do? This is a Final Fantasy game, and all of it's jobs have pulled from old Final Fantasy games. That isn't to say they can't add new unique jobs to the fray, but while they still have iconic and HIGHLY requested staple Final Fantasy jobs yet to implemented it's more strange to think they wouldn't continue to pick from those iconic jobs.
Then things need to be considered from a developmental standpoint. Tanks in this game wear Fending armor, so either future tanks will share this armor, or additional work needs to be done to make a future tank job wear a different class of armor. Not only is that more dev work needing time+money, but it also brings in new complications when it comes to itemization within the game. So going under the pretense they'll keep things on the easier side, because choosing the path of least resistance is often the norm for businesses, then future tanks will continue to share Fending gear.
Once we've established that, then we can look at what past Final Fantasy jobs wear such armor. That's a pretty small list. From that list, what jobs have more requests/popularity, you want to pick a job that has widespread appeal/allure to entice not only existing players to try them out, but new players to buy the game/expansion the job comes out in. So of the small list of heavy armor wearing Final Fantasy jobs, the obvious most popular one is Samurai. It also uses a slashing weapon, which while repetitive in the sense that the other 3 tanks have slashing weapon as well, from a design standpoint it keeps things simple. All tanks use slashing, continuing this trend eliminates the potential imbalance that could come with a tank that deals a different damage type. (This assumes the battle system reworks in 4.0 won't be removing/changing the way damage types works).
All I'm saying is that SAM as a tank logically fits what FFXIV's archetype is for a tank. It's popular/requested, it fits the armor, it fits the damage type, it is also a job with a romanticized fantasy that involves honor and protecting others, the same a knights. It doesn't need to have a history of defensive abilities to be a viable choice for a tank role because it meets so many of the other criteria, the fact it does have a history of at least some defensive abilities certainly doesn't hurt though either.
And I've never said SAM couldn't be anything but a tank, only that it should be a tank. Every job is a DPS. DPS is the default, it doesn't need arguments why it should be a DPS from a design/developmental standpoint because literally everything can be made into a DPS. Tank jobs, not so much, examples of tank-potential jobs in Final Fantasy games that fit FFXIV's definition of a tank are few and far between, and taking one of the best and most viable options for another tank and using it as another DPS removes it from the pool of very few options.