The point is that most of multi-purpose themes can be used to build this kind of themes, but i get your point.
You need an out of the box minimal work to deliver to client theme. Basically you need a theme in which you import demo content and change images and texts, minimal work maximal output. I am all up for that, but you have to keep in mind that this kind of themes are a big risk, yes they managed it but that is not a rule it is an exception.
Once envato implements the search that we need you will start to see much more niche themes, and you will be able to find this kind of themes among already published one. There are themes for this kind of services but its virtually impossible to find in a normal time frame.
And why to people push to multi-purpose themes? Because of the returning customers, from one of those themes you can make almost unlimited variations for range of types of sites, and people get used to the system and just keep on buying so one customers owns like 15 licences and with every update full of features they come back.This is made from one of most popular HTML themes a while a go, http://www.sersentransport.si/ it didn’t take long time. Leave me your contact via our portfolio and i will send you a link soon for an example.
But yes niche themes in my book should be priority, for us its also less work, but sales eventually start to decline, and time frame to that is not big…
We all strive to that one gold mine theme that will bring us tons of sales and not lot of support work as you strive to a out of the box themes for which you can buy them for 45$ and in 5 hours finish and sell for lets say 1000$