It's that old saw of "anyone can create a security system that they can't break". How do you know that your combination of methods is secure? There may well be

It's that old saw of "anyone can create a security system that they can't break". How do you know that your combination of methods is secure? There may well be hidden (or at least non-obvious) complications with using certain methods and/or implementations together that make them easier to break. If you're not aware of that, but someone else is, then the first you'll know about it is years later when a security leak is traced back to your "secure" system. And the only other way of knowing about this sort of stuff is to spend a lot of time staying current with the research. Or you can let someone else do all that hard work, and run the extremely small risk that their work has been silently compromised by the NSA (or other state actor of your choice).