Why does the LastPass installer not offer to import my Safari passwords?
The Safari web browser stores its passwords in the "Local Items" Keychain. Unfortunately, this Keychain isn't stored as ~/Library/Keychains/*.keychain like the rest of the Keychains. Instead, it is stored as ~/Library/Keychains/*/keychain-2.db, which is an SQLite database containing mostly encrypted data. Apple's SecKeychain routines cannot access this data, so LastPass cannot import from it because we are unable to override Apple's obfuscation of these passwords.
Many users have come up with some solutions to circumvent this limitation, however, one that has been tested by LastPass can be found at https://gist.github.com/rmondello/b933231b1fcc83a7db0b.