PGP is like a seatbelt in your car in an era when most cars didn't have them. Just because most don't have seatbelts, it doesn't mean you shouldn't use yours. It may not protect you from all accidents but it protects you from some pretty bad ones all the same.
Getting PGP setup in Thunderbird is no more difficult than installing and configuring a password manager. Basically, install a piece of software, click setup, and click create key. Then when you send an email, click an icon that looks like a lock before you send it.
I agree that it should be easier. I think all direct person-to-person messages and conversations (voice too) should be encrypted by default. The software itself should come with it installed by default as well.
All of the exterior doors on our houses come with locks and keys. Having to separately install encryption software in an email program is like everyone having to install their own locks at home--as opposed to the builder installing them when the house is built. In the end, sending an encrypted email should be the virtual equivalent of turning a key to lock your door when you leave the house. So then the average person can also be oblivious to the make and model of the lock and just be satisfied that it works.