Looking at Heavensward and Stormblood, especially Sohm Al and Sirensong Sea.

By the time a player reaches those dungeons, they will have received at least a set of i110 and i240 gear, respectively.

Every HW/SB dungeon has at least 1 boss that has high unavoidable damage. It is impossible for most jobs to survive such damage on their own with insufficient gear.

If a player runs "The Steps of Faith" or "Baelsar's Wall" which have an ilvl requirement of 90/230 respectively, why does Sohm Al or Sirensong Sea not require at least that much?

With insufficient gear, a duty can be impossible to complete. The Duty Finder can and should prevent this.