Because most email and social media users do not have the notorious modern devices that have a reader. Almost all personal computers do not have readers, and when replacing passwords with fingers, you will have to buy peripherals.
So humanity will not replace good old passwords with a gadget with dubious advantages simply because no one needs such happiness and for nothing.
Because fingers are the most damaged part of the human body and tend to wear off, rub, cut, etc. In addition, they can be easily counterfeited or used without human consent. Passwords, provided they are sufficiently complex, are several times safer, and for one they will not do any good if one of the accounts is leaked or hacked.
A fingerprint has never been and cannot be a sufficient confirmation of a person's identity.
First, a fingerprint is used in payment and when receiving documents, but it only complements the verification process, because the person is personally in front of the verifier.
If there is no person who can personally verify your identity by ID, then the fingerprint becomes not a trusted means