It's called iMessage. It's possible, Telegram just doesn't care. All their differentiating features (large groups, channels, device sync) is directly enabled by the lack of encryption.
they do have encryption, just not e2ee, and in fairness to them, it doesn't make sense to have e2ee on a channel or a group with 100k ppl in it, also device sync is possible with e2ee, it's just a slower
you can have large groups and device sync WITH e2ee, see Matrix.
Any Matrix client I tried lagged even without chats though.