*Every social media app...* - I have worked on warehouse automation projects, worked on multiple "specialists help tools" where logout button or just putting a text box was negligible effort.

Building HN like site - for me solved problem - there is bunch of clones on GitHub you can "git pull" and have it running in minutes. With docker it is even faster.

There is Wordpress, Salesforce you name it, you don't need to have any code and no low code. Hosting providers let you setup whole blog/e-commerce site with a single click.

In MailChimp I can create landing page with working email form without even knowing that HTML exists.

Using any of the frameworks like Rails or Django gives you that 80% or 90% that you describe, login/logout for free no coding. So if you just want to put some buttons and text boxes it is all solved already.

What developers are busy is filling in those 10% gaps that you point out, because it is still complicated and one has to understand things from multiple angles.