Why are the header and footer tags not used instead of ids?

On multiple exercises, I have seen something like this in the code:

<nav class='header'>
<div class='footer'>

Why isn’t Codecademy using a dedicated header or footer tag?

Could you post a link to these exercises?

I can’t post all the links, but here’s where I last saw it.

In the case of the <nav class="header">, it was done because they wanted the semantic value of the <nav> tag, rather than that of the <header> tag.

But, why shouldn’t we nest the <nav> inside <header> ?

You could, but it seems are bit redundant when the only thing inside that "header" element is a nav element.