As the writer of Markdown content, you're often both the user of the content and the publisher of the content, so you do care about how it looks. Even if you're just the user, your intention when writing is to communicate something, and the eventual appearance of your content impacts how it is interpreted by the reader. So you still care about how your writing will look, which forces you to care about how the publishing process interprets the markdown syntax.

When you publish book you have mostly zero power over how it will be designed. And honesty you should have zero power over it because there are people editors/designers/typesetters who dedicate their lives to exactly that. Better to let them handle it. Same goes for the web.

A system that prefers semantic tags such as "emphasis" and bans direct formatting such as "italics" only works if you can import or define a semantic tag for "book title" that you know will be formatted correctly. It doesn't make sense to tag a book title with "emphasis," because there are a lot of different ways to express emphasis, and only one of them works (quite coincidentally) for a book title.

First, I think they could have repurposed the old tags. <i> can stand for "important," with a default style of italic, and <b> could mean "bold" in the sense of "strong" or "outstanding", https://www.etymonline.com/word/bold, with a default style of boldface type. You can still restyle them to your heart's content.

Second, italic doesn't always mean emphatic. Like I said, it is also a formal convention for: titles (books, movies, magazines), ships, foreign words, legal cases, etc. (Only of large works. For articles and other small works, you quote them. So for example, The New York Times should be in italics, but any particular article, like "Chiefs Win Superbowl", should be in quotes.)

Third, the tags are longer, especially <strong> instead of <b>. It's noisy. A quibble, you might say. But so is this whole topic. And there is a line where coding goes from fun to tedious.

Fourth, an asterisk looks stronger than an underscore anyway:

  *this*

  _this_

There can any number things set in italic on page and none of them have to use <em>.

"italic doesn't always mean emphatic" is exactly the reason why i say think of it as emphasis not as italic.

Also emphasis/strong in html is mainly for general emphasis in paragraphs of text. It works pretty much like h1 and h2. The difference is that you have only two levels.

I'm a user and I care a lot about how my documents look.

Of course you are both writer and publisher you are also in control of the design. But when you are writing you shouldn't be thinking about how to present the text. These are almost always different processes (except rarer things like poetry or experimental prose that rely on whitespace). Write > edit > present/design.