The Treasures of Vana'diel thing last year did offer like 4 new FFXI shirts/the Impossible to Gauge hoodie (which is comfy af).
This year is the 30th anniversary of FFVI (which is why they played Dancing Mad and Searching for Friends at Distant Worlds, apparently), so the pixel remaster shirts showing Amano some love and the iconic Terra art don't come as a surprise.
XIV's new expansion comes out in a week, so that's a no-brainer. And XVI is the newest release whose latest DLC just came out in April. It's just a function of timing.
The shirts are already out here in Japan. One of my coworkers is wearing the 16 shirt with the summons on the back today, it looks pretty cool. Definitely recommend if you're into that.
TLDR: Only locals from Japan could order those items. You're in Japan, so your relationship with SE is entirely different.