Probably because the NAND flash memory buried inside the phone is what is commonly known in the tech world as MLC (Multi-Level Cell, typically two layer) NAND Flash Memory
. That stuff is expensive to make. That's why SSDs for computers have been traditionally a luxury item.
MicroSD cards more than likely use TLC (Triple-Level Cell) NAND Flash Memory. It's much cheaper to make but has less life expectancy than MLC NAND.
Up until very recently, all SSDs for computers have been MLC NAND but that changed with the Samsung 840 Series SSD which uses TLC NAND. They say that it has great write life but they have to do a lot of work in Digital Signal Processing to make that happen. And for cheap MicroSD cards, you can bet that "extra work" isn't going to go into those chips.