For all they know, a malicious third-party could be redirecting the connection using another self-signed certificate bearing the same holder name.
Self-signed certificates can be created for free, using a wide variety of tools including OpenSSL, Java's keytool, Adobe Reader, wolfSSL and Apple's Keychain.
They are easy to customize; e.g, they can have larger key sizes or hold additional metadata.
Their use doesn't involve the problems of trusting third parties that may improperly sign certificates.
Developers of web browsers may use procedures specified by the CA/Browser Forum to whitelist well-known, public certificate authorities.
Individual groups and companies may whitelist additional, private CA certificates.
[3] Self-signed certificates have limited uses, e.g. in the cases where the issuer and the sole user are the same entity.