...and in fact, they are purposefully (by default) made to be non-encrypted so people can choose use to use their own encryption. As an example, the internet is not encrypted by default as well - encryption is built on-top of it.
Default encryption doesn't prevent you from adding your own layer of encryption on top of it, if you need it.
People can use encryption for their emails. Any one they prefer. How many actually do it?
The beauty of a built-in default encryption is that, as a user, I don't have to worry about anything. I don't even need to generate keys and do all the other setup tasks, like for PGP.
And another blessing is compatibility. When you have one, strong encryption for everybody - it's good. When you have to use different encryption schemes for each of your recipients - not so much...
This is easy enough to include in clients. In fact, I'll eat my hat if there isn't a client that let's users check a little box (or equivalent) to encrypt their message.