Because most software that is written costs the company who wrote it money when someone has to go back into the code to maintain it. They also have to dig into the code to fix bugs even before the sell the first copy of it.

So when writing software the company that writes it gets benefits from making it maintainable, rather than just making the first version as cheap as possible to produce. Even software that doesn't make it to version 2 is often coded in a maintainable way in the hope that there will be a version 2.

Building and maintaining buildings is very different. The builder doesn't care how easy the house is to maintain. The house owner isn't go to call them back in 12 months time to ask them to add some new network cabling at no cost. They also don't really have much of an issue with having to go back in to fix bugs during the building process.

Making new builds quick to throw up but hard to maintain actually makes more money for the building industry, as when the house owners gives up on maintaining it themselves it is the industry that picks up some extra work.

This is of course turned on its head when the software company involved has rich clients and its own consulting arm

Edit: I spotted one flaw in my argument. New builds of course do come with some sort of guarantees, there there must be _some_ incentive to the house builder to make things easy to maintain when they get a call 12 months after selling that some bit of wiring or pipework has failed.