There is no reason for unencrypted messaging.
This is a fundamental market failure.
It is only through bundling these messaging services with other services + platform dominance that unencrypted messaging still lives.
Unencrypted messaging is easier and more convenient, just login from anywhere and done. So there are actual technical and rational reasons to choose against e2e.
This is why Telegram and Discord are so popular. They are popular because they work well, and part of the reason they work well is not bothering with E2EE. For instance, when you join a group chat, the server can just send you the message history (if enabled) and there's no need to negotiate keys with every other participant. There no "joining...", there is no "message will appear shortly...", you just press the button and you're in.
I've never seen an E2EE group chat that could remain stable with more than a few hundred participants. Even Matrix gives up and just makes it unencrypted at that point.
Many people do, e.g. by switching from Whatsapp to Telegram.
The market is working alright; people are (uninformedly) voting with their wallets (or rather, their personal data).