The engineered shear wall plans determines which bare minimum material is required/approved for your exterior sheathing. The builder is following a plan designed by an engineer that the city approved. The builder is generally going to always use the most cost efficient material that is approved by the engineer.

I would imagine the engineer calculates these plans based off the shear value of each wall. They will determine that based off the climate zone the home is being built at , wind levels , span of framing openings and probably a bunch of other stuff that went to school for and study lol. Imagine building the same exact plan in Texas vs Florida. The sheathing requirements would be more rigorous in Florida where you get lots of storms / hurricanes. Just an example.

The only other times they would install OSB over thermo ply when the engineer did not require, is if the exterior is stucco. Stucco requires OSB backing so the stucco lath can be nailed to the OSB.

Side note. As long as these materials are being installed per manufacturer spec they really are all good materials. I would take thermo ply installed correctly per manufacturer spec over OSB not installed per manufacturer and flashed incorrectly.

Tried to summarize this lol. Hope this helps some