Key Exchange

The hard part of symmetric ciphers is:
How do Alice and Bob share the secret key?
Meet in person
Diplomatic courier

In general this is very hard.  In the extreme, we have the One-Time Pad.
It's a cipher with provable security (and that's very rare!)
The problem is, the key is as long as the message, and you have to use a brand-new one for each message.
So it's not really usable in practice (without diplomatic couriers, anyway)

Or, we invent new technology...