First probable reason is that we have been picking up the toys for them for their whole life. So why should they start now?
Second probable reason is that we don’t pick up our stuff ourselves. We don’t really care, deep-down, that there are toys everywhere because it is easier to just leave the mess than to nag about the mess.
Either way, we need to talk. And I don’t have a magic, easy answer for you, dude.
There are 15 parenting books on my bookshelf, over there, right now!, that will tell you how important follow-through is and how healthy boundaries make healthy children.
The same principles apply to being organized: Set a good example and be consistent. Once you are in charge of little people, this becomes doubly important. And I know it isn’t easy, people. Having children is freakin’ hard. That’s right! I said FREAKIN’!
Once they get in the habit of putting things away it’s like washing their hands. Sometimes you will have to send them back in to do it. But, a lot of the time they will do it on their own. Help them develop the habit.
For more ideas and specific details see this older post on organizing for kids.