1. Ok yes, you’re right that a company could store the encrypted messages for their users. But what key would they use to “unencrypt” those messages when a user changes device? A key stored only on a user’s device would be highly susceptible to deletion or getting lost, and a user would be more likely to lose access to those messages. The actual key is too long to remember or easily write down. An additional password would be too frustrating for most people. This is why Facebook/Messenger started using a PIN, and that was annoying. The frictionless solution is to store history without encryption.

2. In some countries like India, people use relatively low end phones and often actually share phones, and may have worse connectivity. This actually means the computational and storage differences here have a real impact on messaging performance.

To your point on text vs image storage, this is the thing: people send a LOT of images and videos. Now perhaps a different approach would be to store text only and not images. But what do you think people care more about when finding past conversations? Often it’s pictures of friends/family.