Why does IMDB use full page Postbacks for Navigation?

I noticed that IMDB requires a full page postback to navigate to a new page. This is particularly strange to me because I have always been told that full page postbacks are wasteful - given that there is always some common markup that you can save by avoiding them.

For example, in case of IMDB, the header and footer contents are (almost) always going to be identical across two pages that you browse. So it would (normally) not make sense to resend that set of markup on every request.

One of the reasons I can guess is that the site caches entire pages. This would at least make sense for cases when user navigates to popular pages (movies, shows or actors) given that they are likely to be in the cache - resulting in faster response.

However, the benefit of such caching can still be availed using templates (such as those in React JS.) So, the reason to not use partial postbacks still eludes me! Particularly so, because Wappanalyzer on my Chrome thinks IMDB uses React JS!

I believe there would be a good reason for the way IMDB works. So the only question is, what am I missing? How are full page postbacks technically justified?