There's a lot less unpredictability in building a house than in writing a program.
There is easily an order-of-magnitude difference in how productive different software developers within a typical organization are, and probably almost as much difference between software development organizations. Building contractors do not have that level of variability.
Hourly rates are probably also an order of magnitude different between some little town in Flyover County and San Jose.
On top of both of those, what infrastructure are you assuming for the software projects, and what performance, reliability and security requirements do you have for your software? Any of those variables can radically change what a cost-effective development approach would be.
Finally, most custom software projects involve a lot more novelty than a house. Just about anybody can figure out how many square feet of marble are needed for a countertop, but it is quite hard to estimate (within a factor of two or three) how much effort it will take to develop a feature that the developers have never tried before and that the customer can't thoroughly explain.