There’s no easier time for a builder to install finishes than when they’re in construction. They don’t have to remove existing finishes to replace them so it should be cheaper for the builder to do it then – and it is.
The problem is that those savings don’t get passed down to the buyer. In fact, the builder charges a lot more to do it than the the homeowner would pay outside contractors to do the install for them.
A construction lender could probably do a loan whereby the buyer takes possession of the property after the shell is completed and the costs to finish the interior are fund-controlled while the buyer’s contractors finish up the interior; then the total can be financed under permanent financing. It’s like doing a construction loan, except instead of starting at ground zero they’re starting out with a shell. The interior buildouts of commercial and industrial buildings are completed this way all the time, and there are a lot of lenders who know how to structure such a deal.
Just think what a builder would have to sell the shell for if the buyer had breakdowns on the pricing on the interiors of other units already sold; the buyer could deduct those “costs” and start negotiations from there.
If a builder is desperate to move inventory they’re liable to do anything.