Telegram's E2E encryption is not secure. See: Is Telegram E2E Still Insecure? and Is Telegram secure?
Lets asume its is secure and talk about the "excuses" why E2E encryption is not always used. It is only used in secret chats, from ONE device to another ONE, but not between all devices from one account to another. Groupchats are also insecure.
Is the key exchange between two devices with the same account really so hard to implement, that it is still easily usable for everybody?
Another excuse is, that bots could not read chats, when they are E2E encrypted, but couldn't it be implemented?