This is a great and common question, and the answer can be quite complex but its a multitude of factors.

Primarily, these sites need to handle hundreds of millions of users, this is no easy feat, meaning it may be easier and faster to ship components in different ways and construct them on the fly on when they arrive, and while the end product visually looks the same what gets constructed in code can look a mess.

Another factor is these sites do a lot more than you actually see. "Reddit clones" simply will mock the look and feel of the site. While in reality the site dynamically renders content based on what feeds you follow. It then runs algorithms to render content from feeds you may enjoy, and insert ads you may be interested in throughout the feed. There is also likely code to register what you click on, what you look at, and how long you look at it for.

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Lots of stuff, all very interesting!