As a number of people have mentioned, compatibility is one reason. However, there's another reason, particularly for flash drives: the FAT itself. Flash drive

As a number of people have mentioned, compatibility is one reason. However, there's another reason, particularly for flash drives: the FAT itself. Flash drives tend to be MLC or TLC NAND flash, which is slow and can sometimes be error-prone. Oftentimes manufacturers will perform a trick and have the start of the disk actually be faster SLC NAND. Then, as a further enhancement, they'll make sure the table falls on sector boundaries. With a more complicated filesystem this becomes more of a challenge. But with FAT, it's easy.