Tiles and paint, maybe.

But cabinetry usually subcon out to cabinet specialists, so developer won't have the materials

Built in cabinetry that come with projects are usually modular type with standardized sizing, they'll cut the parts off site and and then only delivery what they require to the project site. More cost effective this way and can make in bulk.

Unlikely they'd have pieces of board lying around for extra to use, because they can also just use the boards in future projects since most houses these days are standard sizing.

Honestly if you want to DIY cabinet and you have the carpentry skills, just buy materials directly from wood supplier. You don't have to deal with any profit margin trying to buy from the cabinet specialists.