Because currently they have search and they do user-to-user messaging, good luck implementing that over the web in a reliable and scalable way with E2E encryption.
>>embedd+(OP)
I'm not sure you understand the e2e encryption, the search can be ran on client side after decryption, there's no need to run it on the server side.
The chat is a bit more transient but you're transferring messages encrypted again over websocket for example, then decrypted on client.
I'm not seeing what the blocker is, or even issues with scale given its the client side doing the work.