This is incorrect. You can sure write LaTeX that is intricately dependent on the output dimensions. But you can just as easily write LaTeX that is independent of output dimensions.
Case in point is compiling LaTeX doc to HTML which you'd admit is easily resizable.
Case in point is also writing LaTeX docs for journals or publication where you can easily resize the document to match the publisher's style guide and dimensions by changing the documentclass.
Using styles means that you use someone else's backed in page size settings and still need to recompile when you change to a new style.
The TeX philosphy rejects that. When TeX can't format a paragraph beautifully, it emits diagnostics like "overfull \hbox".
That's totally incompatible with being able to dictate a width, and expecting things to fit without having to get involved.
The dimensions of all the documents I've written over the years depend on the document class I use, and as our company keeps changing document layout standards every few years, the document class changes as well.. but the only source editing necessary is to replace the very first line in the document, i.e. changing the document class. And the whole document changes. Sometimes completely, much more than just sizes.
But, yes, PDFs are intrinsically non-resizable.