One of the stated goals of the RPi foundation is to keep the price as low as possible. Adding a few GB of NAND flash adds ~$5 to your BOM for the part, plus yo

One of the stated goals of the RPi foundation is to keep the price as low as possible. Adding a few GB of NAND flash adds ~$5 to your BOM for the part, plus you have to redesign and solder that extra component to the PCB. All told, it would probably increase the price of the board by ~$10, and cause suppliers to have to maintain additional models of boards in their inventory. If you want a board with storage built in, look at the BeagleBone Black or the CubieBoard (there are several others as well). The communities for these boards aren't quite as large as the RPi's community but should still be large enough that you can get help doing whatever you need.