New home with gaps in stairs
Those look like temporary steps to me. For carpeting you usually see 1-1/8" bullnose particle board treads and 1x8 or 3/4" plywood risers. 

Rarely is two-by lumber left as a final product, as it's heavier-duty than necessary and creates very thick and squarish nosings when carpeted. We'd commonly do something like that, or doubled scrap plywood, until after taping and painting is done, then install the final materials using construction adhesive. 

Yes, it should be done better, but that's my opinion. It really comes down to your relationship with the builder and your contract. Ask your builder if that's the final state.