Because it's too expensive and bulky for bulk-produced custom devices based on the Sparkcore.
If you read the spark.io home page, near the bottom you'll read the following (se the part I marked in bold) 
Built to scale
The Spark Core is a development tool, designed to get your project off the ground quickly. But what happens when your production volume scales to thousands? The Core is ready.
We use industry-standard components — chosen to be powerful, inexpensive, and widely available — that can be designed directly onto your own circuit board, bringing your Bill of Materials down for volume production."
I have already made a one finished project, where I just soldered the entire Sparkcore onto my custom PCB. It was the best choice for this one. But I have other ideas in mind for smaller projects, where I'll need to compact things down. If anything I ever do becomes a product that others want to buy (unlikely, knowing me!) then doing this will help the costs considerably, so long as I'm producing enough of them in each batch.
So I want to learn how it's done, by practicing it in real life.