RSA, DES, AES, etc. all use (relatively) complicated mathematics to encrypt some message with some key. For each of these methods, there have been several documented vulnerabilities found over the years. Some even believe that there is a vulnerability in AES known only to the NSA.
On the other hand, the One-time pad is not only ridiculously simple, but also has been shown to be impossible to crack (if used appropriately).
Say you have some message to be encrypted with length n. Simply generate a random string with length m where m is at least as large as n, and perform modular addition of the random string onto the message.
As long as the random string is generated with a high-enough quality random number generator, and as long as the same one-time pad isn't use on multiple messages, it should be impossible to crack.
If this is true, all we would need to perfect are fast, cryptographically secure deterministic random number generators. Use the key as a starting seed for the RNG and you have a powerful uncrackable encryption scheme which is simple to understand and implement.
Blum-Blum-Shub is a fast, cryptographically secure PRNG, and there are others too.
I have written a C++ program which uses two 2048-bit primes (which match the necessary criteria) for Blum-Blum-Shub and uses a password which the user enters - converted into a starting seed - to run byte-wise modular addition. The process is fast, deterministic, and if the literature I have read is correct then it should be very secure.
So why is so much time and effort invested in coming up with convoluted encryption cyphers when mathematics shows that the simplest is often the best?