#openpgp traditions and #signal both bind a cleartext identifier, phone number or email address, to a cryptographic key. It opens up attack vectors as the servers/orgs controlling this binding can interfere.
#deltachat avoids such cleartext identity bindings by creating random #chatmail addresses, as transport only. The cryptographic key becomes the identifier and we want it hidden from the transport layer. Only people being in end-to-end encrypted chat need to identify each other, after all.