Because doing proper crypto is VERY hard. You might think you've gotten the ultimate security and one year later someone will defeat it (or cryptanalize within

Because doing proper crypto is VERY hard. You might think you've gotten the ultimate security and one year later someone will defeat it (or cryptanalize within a practical limit, which boils down to the same), because you forgot a detail. Even just implementing a crypto algorithm properly in a way that doesn't leak information is very complex, reason for which most applications tend to use well-established crypto libraries.