Why not the one-time pad with pseudo-number generator
It's a good question. As pg1989 said, this is the basis behind stream ciphers, which are very fast in practice.

I thought I'd quickly expand upon your statement that "the one-time pad is the perfect cipher and impossible to crack." This is true, in a sense, but it's worth pointing out that sometimes an attacker wants to do something simpler than "cracking" the encryption scheme in order to recover all plaintexts.

For instance, an attacker may just want to change the message so that the honest recipient decrypts something else. As a simple example, suppose that the attacker knew for sure that the message was either "yes" or "no!" and just wants to change the message to the other value. Note that:


This is simple to do for messages encrypted using a one-time pad. The linked Wikipedia page describes why this is true, but it's probably more fun to figure it out for yourself.
The attacker can do this without learning the plaintext. Hence, the original claim of a one-time pad being uncrackable is still true.


There are some other issues with a one-time pad construction too, such as the fact that usually some state will be required to make sure that you don't re-use the seed during the next encryption. Hence, while your question is definitely not silly, and stream ciphers are very useful in certain circumstances, they also have their limits.