I mean it copies all the Stats apart from HP, so why does it need other stats?
Simple answer. All Pokemon Need stats. These stats are used for calculations. If Ditto didn't have stats and it was battling another Ditto no damage would be done at all. Also if the Pokemon ditto transforms into runs out of PP then struggle would have no effect because it has no stats.
A question on the DB a bit like this
Hope this helps ^.^