Two logical reasons:
1. The main reason for defrag on a spinner is that most of the slowdown on a spinner that needs it is caused by the movement of the read/write heads. If a file is spread out over multiple tracks on the drive, the heads have to move all over the place to get the full file read. Same for writing, if the heads have to move all over the place to find space to write a file, that movement is much slower than the data transfer. On SSDs there are no heads and the reads/writes are all electronic, so the benefits of defrag operations are practically nil. (There is also an effect of overloading a drive beyond about 85% that has to do with the rotational speed and data density that you avoid with SSDs, so the old adage of not filling the drive beyond 80-85% isn't as impactive with SSDs as it is with spinners, either.)
2. SSDs have limited numbers of "writes" in them before they start to fail. The limit is pretty high, and you are not likely to hit that limit before you decide to trade in the MBP, but defrag involve LOTS of writes, so the thinking is that there is no reason to use up writes for very little gain in performance.
