Enabling E2E encryption in Telegram limits you to only the device that the chat was created in. WhatsApp and Signal allow you to use their desktop apps (not the best app but that isn't the point). This allows me to use a full sized keyboard when I have the need.

It's not complicated, but it's still something you have to manually do (meaning many will never do it because they don't know it exists) and you lose a lot of features that seem to be the selling points of Telegram