Why are plain static websites not enough for modern web applications?

Question

Accepted Answer

Because real applications need a back-and-forth between user and system, and a static site can only display fixed content, never react to input.
A static page is a one-way street: it shows you what its author baked in, and that is all. Modern services are defined by the opposite — the user does something, and the system responds with a result tailored to that action.
Typical interactions a static site simply cannot provide:

Search -> Result (e.g. a product or flight search)
Input -> Storage (e.g. submitting a contact form)
Order -> Delivery (e.g. checking out in an online shop)
Profile -> Personalization (e.g. Netflix recommending shows for you)

The shared goal across all of these is enabling user-system interaction over the web. To get there you need ingredients a static site lacks: server-side programming (PHP, Python, Node.js) to process requests, a database to remember things between visits, and often client-side JavaScript so the page can react instantly.