Why don't most sources classify "when", "where", and "why" as relative pronouns?

I am researching the use of relative pronouns and most websites, including the British Council, list only:

who, whom, which, that, and whose

What about here?

That's the house where I grew up.

January is when we go on vacation.

That broken window is why you are grounded.

Notice how what is also missing from that list.

What she said is true = that which she said is true.

They apparently only include relative pronouns that usually refer to an explicit antecedent, as opposed to those that usually include their antecedents, as do what (always) and where (usually). It is just a matter of definition. In your examples, those words clearly function as relative pronouns.