Lets step away from the front end for a second, on the server side you use a reasonble MVC framework.
* You get to stop worrying about how to structure your code.
* The guidence should use Depency Injection, allowing you to easily Unit Test.
* You get a templating language that helps with XSS.
* You get a validation language that helps with input.
* You get session handling that projects you against jacking, etc.
* You get an ORM that makes SQL easy and projects you against SQL Injection.
* You get Security Headers, etc.
Typically, the junior developer needs the above help, the mid level developer thinks they can do it all himself, and the senior doesn't think it's worth reinveting the wheel.
JavaScript has arguably been the wild west, and if you're creating a SPA, a lot of the above is no longer your consideration, but it alos doesn't neccessairly mean we should all write everything from scratch.
Now there are counter arguments where you can DIY your own framework using community modules, and this is how the Golang community seems to work, but this seems better when you have something like PHPs PSRs ala https://www.php-fig.org/ defining how this should fit together.