It’s end-to-end encrypted, you only need enable secret chats for that. They are disabled by default because Telegram team is smart enough to give people proper chats syncing (which the majority of users care more than end-to-end encryption). By default, Telegram uses encryption too, of course.
Other messengers, such as WhatsApp and Viber have chats “syncing” with end-to-end encryption at the same time, making the whole idea a scam.
If someone says “Telegram is not encrypted” or “Telegram does not have end-to-end encryption”, simply put: it’s a Facebook propaganda