Why doesn't the compiler complain when something implementing a serializable interface isn't itself serializable?

Are you referring to something like this?

@Serializable
sealed interface Person {
    val name: String
}

data class PersonWithAge(
    override val name: String,
    val age: Int,
) : Person

A compiler error here wouldn't really be appropriate, since the above code represents a case where you specifically don't want a particular subclass to be serializable. Perhaps a warning would be more appropriate, forcing the developer to explicity annotate their intention to have this subclass remain unserializable in order to suppress the warning.

Yes. If I do Json.encodeToString(Person.serializer(), PersonWithAge()) it will crash