Speaking as someone in the NAND industry...
NAND does not have its own reliability controls on-die. Items such as wear-leveling, file management, and ECC mechanisms need to be handled somewhere. So the options are in software, which would then need to be validated and designed for each NAND manufacturer, die, and process; and would consume CPU and batter power from the tablet OS, or it can be done via a separate off-die controller.
And as to the choice of eMMC, it's a cost/performance/reliability trade-off. eMMC is relatively inexpensive (very small die), and includes all of the aforementioned reliability mechanisms at a low-power, and low-cost method, in an I/O language supported by most mobile architectures (SD/MMC). However, it severely lacks in relative performance to an SSD. The other option is an optimized SSD controller, which may cost many times more, but has much higher performance. The problem is how to include a $100 SSD in a $100-200 tablet BOM... impossible.