SD cards are available with up to 256GB of storage. They just cost $400 or so[0], so I don't think enough people see the value of paying for that much storage in a phone.
The flash memory chips used in phones are essentially the same as those used in SD cards, and phones often use SDIO for internal communication. The SD card is just a point of reference for the cost and availablity of flash memory unbundled from a phone.
[0]http://www.amazon.com/Lexar-Professional-256GB-Memory-LSD256...