Why can I only add friends using QR codes and links?

But such link may be intentionally auto-corrupted or completely removed on (government) server side, thanks to immutable i.delta.chat part… one more reason to make DC apps pay less attention to domain part.
See Single link: account creation + chat invitation (+ more crazy stuff)

Username part of address, if long enough, may be used as public key. Tor onion addresses and some messengers do it.