Well... I'm agree with Mico when he said your question involved two different issues: on the one hand the way fonts are scaled to generate the visual structure of a document and on the other hand the (limited) set of sizes available in the standard classes.
I will start answering to the first without relying on what has been said in previous answers and comments.
The percentages given by Mico are a sample of how a typographic scale works, in the case of TeX, is parameterized as a function of a single variable or parameter, which is the size of the base text, however takes into account that the proportions of the scale depend on optical considerations that vary with each typography, inasmuch as their own proportions and strokes may need slightly different adjustments, E.G. a large x height implies ascending strokes shorter than a letter with an x height smaller and that fact can make that two typographies of the same size require different adjustments to obtain the same visual result.
I understand that TeX and LaTeX, in addition, are designed in the best editorial tradition, so Donald Knuth was based on the tradition of the Monotype metal types as Barbara Beeton points out.
In this sense, it is necessary to see that good editorial design does not have a quantic or continuous approach, it is a question of making use of a congruent way of a scale that allows the size of the page, the type box and the typography chosen to harmonize each.
When, instead of using LaTeX, you make use of some DTP like Scribus, InDesign or QuarkXpress (does QuarkXpress still exist?), you need to choose the base grid around which the base body text size must fit for the reading text, and each one of the headings, the text in quotations or footnotes. You must adjust the dimensions of the typographic box so that it matches the final size of the page and that the font size is adequate to form lines of text of the necessary length and fit with the above requirements I have just outlined.
Any change in these decisions may involve a complete redesign of everything, and since this approach is WYSIWYG, it requires a lot of time to make all these changes, in addition to greater knowledge to achieve a result comparable to that achieved with TeX.
In this sense we can say that TeX is a tool rather to design parametrically, and therefore allows to make changes with a facility that people who live in the visual logic of the DTP can't even imagine.
The connection with your second question is on the scale or scales used in design, a very interesting example you can find in Le Modulor.
Since TeX makes use of a parametric design, it is not surprising that the scale it uses is a function of a single parameter, that is, the base body of the text. In this way, by choosing that value, you can adjust the entire scale without having to worry about doing it manually as in a DTP.
I don't know what you think, but IMHO, this is wonderful and allows to obtain a professional typographic quality to any n00b who has neither the time nor the desire to delve into these details.
However, standard classes allow you a value for the base text of between 10 and 12 points, it is not a willful limitation or the product of a technical limitation almost impossible to solve, it is, again, part of a deep-rooted editorial tradition, which dictates that the base body for a book, ranges from 10 to 12 points, depending on the typography chosen and other considerations such as line spacing, width of typographic box, etc.
I must confess that when I saw the first comments to your question I was surprised that no one mentioned KOMA Script, again I thank MICO that he has finished with that omission.
Briefly, I have to say that KOMA Script is a bundle of packages that IMNSHO, greatly improves the capabilities of standard classes, corrects their errors and therefore escapes their limitations. You can choose a font size for the base text smaller than 10 pt and much greater than 12 pt retaining the scales, which you will find, I think, quite useful.
Thinking in the example given by samcarter in the comments to your question, we can get something like this:
\documentclass[fontsize=14pt,paper=a4,DIV=calc,BCOR=5mm]{scrartcl}
\usepackage{lmodern}
\begin{document}
\section{Test}
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do
eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad
minim veniam, quis nostrud exercitation ullamco laboris nisi ut
aliquip ex ea commodo consequat. Duis aute irure dolor in
reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla
pariatur. Excepteur sint occaecat cupidatat non proident, sunt in
culpa qui officia deserunt mollit anim id est laborum.
\end{document}
In a nutshell, as you can see, TeX, and therefore LaTeX, are designed according to best editorial practices, one of them is to use a 10 pt to 12 pt text size for the reading text in a document, from which all the other sizes you can use are calculated. Thus, unlike a DTP or a word processor, you forget to waste time adjusting margins and font sizes, not to mention the format inconsistencies.
\fontsize{13pt}{14pt}\selectfont? Although that's not the optimum.