Before you send an encrypted (PGP-style) mail to someone, you need their public key. The recipient's public key is used to encrypt the message for them. So when you are able to send en encrypted e-mail to someone, they probably already have everything configured.

I guess most people don't care too much about their e-mail privacy; or at least don't have a clue that there is something that could be protected, but isn't. And if you use a free webmail, there is no point in encrypting your messages (and I don't know if it is even possible). If you are OK with Google company reading and archiving all your e-mails... yeah, Google would never do anything evil. ;-) And Google is probably better than Facebook, and many people don't mind sending their private data through Facebook messages.

For many people the costs of encryption would be not only configuring their e-mail client, but first installing it, and accepting that they cannot send e-mails from any place, but only from their own computer. Some people don't even know that it is possible to use e-mails without connecting to a website.