It's not even remotely uncommon to run into players running Doma Castle or Castrum Abania while still wearing some pieces of gear from Stormblood. This game awards exp MUCH faster than it awards gear, so unless you specifically grind for gear of appropriate level, or purchase it with gil, you WILL eventually begin to lag behind on ilvls. As noted, falling behind on ilvls can result in long, difficult runs of what should be faceroll-easy trash dungeons.

Since folks often want to rush to 70 as fast as possible, they rarely bother to grind dungeons for gear, and are often low on gil as well (and often have little patience for tracking down the NPCs that could sell NQ gear cheaply).