There are a few a real problems with encrypted email. Spam detection is much more effective when you have access to the plain text of email as you can both bui

There are a few a real problems with encrypted email. Spam detection is much more effective when you have access to the plain text of email as you can both build out better models of "this is spam" based on your entire customer base marking things that you miss or get wrong. You also get to more trivially go "oh this email is only (say) 60% likely to be spam, but it was also sent to 200k people who have no relationship, so maybe it's closer to 90% spam". The real killer is financial, all the "free" mail providers are making money from hosting your email. Gmail is especially egregious in this regard, and scans the content of inherently private communication in order to build out it the google surveillance infrastructure. Gmail's scanning is why companies like amazon no longer include actual order information in the emails that they send out. A true E2E mail system (in which email was decrypted on the client side), would run counter to the googles goals for gmail.