Static sites are great, they allow you to build a website using nothing but client side tech, and let you host them easily in places like AWS S3, Github/Gitlab pages, or even on an arduino as they dont require any server side code. However they do have one big downside, user interaction.

The term scalability often gets thrown about when creating online services. Most people just assume it means having the ability to add additional servers on demand, and to an extent that is mostly what it is, but of course if you are in a situation in which you have the need to add more servers all you are doing is moving the bottleneck further up the line.