The reason is that flash drives do not have a read/write head, so if you were to defrag one, you wouldn’t get any extra performance benefits from it whatsoever. Basically, without a read/write head, flash drives don’t have to work any harder to find your files. Your files can be laid out in any order and your flash drive will still be just as fast when finding them for you.
Another reason is because flash memory wears out over time. Basically, the more you write (saving files, etc.) to your flash drive, the shorter its lifespan will be. And since defragging is a huge write process, every time you do it, you’re adding thousands of extra writes to your device that just aren’t necessary. So, if you’ve been continuously defragging your flash drives, you should stop. If you want your flash drive to last you a long while, simply stop the defrags. You won’t get any benefit from it and you’ll just end up with a broken down flash drive for no reason at all. Go and defrag your hard drive instead!
The difference in defragmenting a hard drive and a flash drive is that in the case of the hard drive defragmentation can minimize wear-and-tear on the drive, whereas with the flash drive it causes more wear-and-tear.