Because passkey managers have no idea what a service is using its passkey for. They could warn that deleting a passkey could make all sort of bad things happen, but for most services it will be only the loss of access. What the alternative could be? "Before deleting this passkey you must contact this site and ask them what data you will loose. I give you a week. Come back here a week from now and confirm your desire to delete this passkey. I will not make you delete it before that day. See you!"