If your page is just static, e.g. no login, no interaction, everyone always sees the same thing then it scales easily. Scaling means you copy the site to more servers. Now imagine a user adds a comment. Now you need to add the comment to every copy of your site, so that everyone sees it regardless of which server they use. So a comment creates more work the more servers you use. And this is where scaling becomes a complex science, that you need to manually prepare for as a software developer. You need to figure out what data will be stored where and accessed how.