Contractors who do well without being tied to agencies, are not that easy to hire full time. They have the network and the skills to find/retain work, get paid decently for it and still have some freedom. It's not about you retaining the option to hire them full time, it's about them retaining the option to potentially work for you full time.
All those are great, but a healthy dose of luck wouldn't go astray either. Some people just have the ability to be at the right place at the right time, others don't (some people are naturally more lucky). It is difficult to become highly successful without chance playing at least a minor role.
I guess like with everything, it's about practice. It's difficult to be good at something if you never do it. It is a bit of a pain since you never actually HAVE to do in your normal work. It is however worth practicing if you know you're going to be put in a situation where this will be an issue (such as an interview).
If you practice enough, you will get better at it and I do believe that it is certainly better to practice on your own rather than failing multiple interviews and using that as 'practice' :).