It's not really a matter of having the know-how to do something, your argument is similar to the statement “if you know how to grow food of any kind then why do you go to the grocery store to get some?” it's simply an irrelevant factor to the issue. Whether or not you can re-make it yourself doesn't matter, it's purely an issue of convenience for which cracking someone else's code is much more time efficient and economical then re-writting the entire thing yourself.