You only need regular client side encryption to protect things in the cloud. Just generate a key on the phone and keep it there. Probably the reason they don't

You only need regular client side encryption to protect things in the cloud. Just generate a key on the phone and keep it there. Probably the reason they don't do that is that it would make it so that a lost phone would result in the loss of cloud stored data. People store things in the cloud to prevent that sort of loss. Email when encrypted end to end in the traditional ways is safe on the IMAP server but also has a requirement to back up a key off the device to prevent loss of old emails on loss of device.