Most parents think that keeping a room tidy comes naturally to everyone. However, that's rarely the case. Even adults need help with learning to properly keep things neat and organized.
Rather than blaming your kids for creating a mess, a good idea would be to educate them about how to go about keeping everything organized. Think of it as a skilled activity which cannot be done effectively without proper training. I am sure if you help them learn the art of tidying up, they will be happy to work with you on this and slowly it will become a habit. For example, if you can work with your kids to arrive at a method of how to put back toys and where, and over time it will start coming to them naturally.
Also, set an example by first keeping your own room clean and organized which should become an inspiration for not just your kids but also other family members.