That's because the point of markdown is to be an overly simplistic styling convention that reads just as cleanly in clear text (eg on the terminal) as it does in formatted renderers. Tables weren't even part of the original design of markdown. In fact there's a lot of stuff that Github and others have bolted onto markdown.
The real problem with markdown is that now it has now reached the kind of critical mass where people start using it everywhere and reaching for it even when it doesn't make sense simply because it's the first thing that pops into peoples head.
So, in my personal opinion, if you start needing the kind of design sugar as formatted cells then you really should be using any one of the plethora of other document formats out there because you're requirement no longer fits around the core principle of markdown. A great many of which can still be composed in a text editor and some of which are still relatively easy for a human to parse.
[edit]
Getting downvoted on this so I'll provide some citations:
> "[Markdown's] key design goal is readability – that the language be readable as-is, without looking like it has been marked up with tags or formatting instructions"
> "A Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions. While Markdown’s syntax has been influenced by several existing text-to-HTML filters — including Setext, atx, Textile, reStructuredText, Grutatext, and EtText — the single biggest source of inspiration for Markdown’s syntax is the format of plain text email."
> Markup offers an alternative means to encode this signaling information by overloading certain graphic characters (see, e.g., [ISO646]) with additional meanings. Therefore, markup languages allow for annotating a document in a syntactically distinguishable way from the text, while keeping the annotations printable.
I totally agree with you. Markdown is nice as a pretty simple documentation format. It is a good fit for something like a letter, a README or a short tutorial. I you go beyond this level, you are better off with something more powerful like AsciiDoc.
From that point of view, tables should have never been part of Markdown.
Nit: tables are part of github-flavored markdown, not John Gruber's original Markdown.PL, and there's also another table extension syntax for markdown. Apart from that, the point of markdown is that you can drop to plain inline HTML/markup or markup blocks when markdown short syntax is insufficient, such as for complex table layouts. Regular markdown syntax really is made for plain text editing. As such, it would probably make more sense that a visual editor, cool as it looks, targets robust markup serialization (HTML-in-markdown) rather than producing markup shortform syntax IMO (but then again, the plethora of markdown editors may prove me wrong on this one).
It seems like the OP would be better served by CommonMark or any of the alternate implementations which are designed to be more feature complete to solve these kinds of formatting problems.
Regardless, if you want to go down the rat hole of exactly what is Markdown, the only "correct" answer is 'whatever Gruber's 'markdown.pl' does, warts and all'.
I don't think you're getting downvoted due a lack of sources.
Markdown has clearly been extended beyond its original intention, so the criticism on its inadequacy for tables is justified in that it is the de facto standard across platforms and folks are struggling with it.
Whatever its original design goal was is sort of besides the point now. People dislike having to type complex tables manually and not using Markdown isn't a choice in a variety of situations. That frustration is justified.
But tables in LaTeX are not fun, too. Once upon time I was about to write a bunch of big and complex tables with LaTeX and needed to include "mini pages" in table cells to be able to render complex content. It was so awkward that I stopped using LaTeX.
The real problem with markdown is that now it has now reached the kind of critical mass where people start using it everywhere and reaching for it even when it doesn't make sense simply because it's the first thing that pops into peoples head.
So, in my personal opinion, if you start needing the kind of design sugar as formatted cells then you really should be using any one of the plethora of other document formats out there because you're requirement no longer fits around the core principle of markdown. A great many of which can still be composed in a text editor and some of which are still relatively easy for a human to parse.
[edit]
Getting downvoted on this so I'll provide some citations:
> "[Markdown's] key design goal is readability – that the language be readable as-is, without looking like it has been marked up with tags or formatting instructions"
Source: https://en.wikipedia.org/wiki/Markdown#History (Wikipedia, Markdown history)
> "A Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions. While Markdown’s syntax has been influenced by several existing text-to-HTML filters — including Setext, atx, Textile, reStructuredText, Grutatext, and EtText — the single biggest source of inspiration for Markdown’s syntax is the format of plain text email."
Source: https://daringfireball.net/projects/markdown/syntax (John Gruber, one of the co-creators of markdown)
> Markup offers an alternative means to encode this signaling information by overloading certain graphic characters (see, e.g., [ISO646]) with additional meanings. Therefore, markup languages allow for annotating a document in a syntactically distinguishable way from the text, while keeping the annotations printable.
Source: https://tools.ietf.org/html/rfc7763 (original RFC)