In that case, why are people not using meaningful names for addresses?
With a private key, you can easily calculate the associated bitcoin address.
But with a bitcoin address, there is no way to find out the associated private key.
So, in order to create a special vanity address (eg. starting with 1bitcoin), you need a software (eg. Vanitygen) to do the following 3 steps:
1. Create a new private key.
2. Check if the associated bitcoin address matches the pattern (in this case, starting with 1bitcoin)
3. If yes, stop; if not, go back to step 1.
The longer the pattern, the more time and computation power you need on average.