Secondly, it could easily be argued that a legal extradition route would be a lot easier than secretly kidnapping and disappearing people over the border. Why do you think it would be easier to kidnap?
As for what's easier, let's look at it this way. For me to kidnap someone, I can just find where they are staying, break into said place and then take them away; if I am super careful, I will do it when there is no one around. For me to create a legal extradition route, then I would need to look into your history, financial dealings etc, find out somewhere that looks dodgy, and try to use that as evidence; sometimes I cannot even use that as evidence given it may not even be committed in China. Remember, Al Capone was sentenced not due to the other crimes he committed (due to lack of evidence) but rather due to his tax evasions, so proving something legally when you haven't done it is very hard. If given that you can just go around "kidnapping and disappearing people" as you have stated, why would you want to bother create the legal route right?