It is under certain circumstances. Specific to ProtonMail, it is E2E encrypted if you send a message to another ProtonMail user. They also have a feature where you can send an encrypted email to an outside address. I think in that case the recipient gets a link where they can then input the decryption password to read the message.
But you’re right about any email you receive (from a non-ProtonMail address). Those can not be E2E encrypted and are only stored encrypted at rest.
You are not Google’s customer. You’re the product they sell to their customers.
Edit: Made this comment before refreshing the post to see the person above me. What they said.