Plain HTML is super verbose for basic text formatting. Having to wrap every paragraph is super annoying UX.

HTML5 is not all that verbose. You can omit closing tags for many elements (for example, </p>, </li> and </td>). Many other tags are also implicitly opened, like <body> and the like. I actually prefer it to markdown for most cases.

Is that a spec or it just happens?

Part of the spec, and recommender by many people.

For some examples of modern html, see google's style guide for html, section 3.1.7 shows an example of a complete and correct document that may blow your mind. Also, the examples of tables and lists without closing elements:

https://google.github.io/styleguide/htmlcssguide.html

It sounds good in theory, but in practice it adds a huge mental overhead.

> A p element's end tag may be omitted if the p element is immediately followed by an address, article, aside, blockquote, details, div, dl, fieldset, figcaption, figure, footer, form, h1, h2, h3, h4, h5, h6, header, hgroup, hr, main, menu, nav, ol, p, pre, section, table, or ul element, or if there is no more content in the parent element and the parent element is an HTML element that is not an a, audio, del, ins, map, noscript, or video element, or an autonomous custom element.

https://html.spec.whatwg.org/multipage/syntax.html#syntax-ta...

This is so complicated. In the end it's easier for me to close the tag instead of second guessing if I may skip it or may not.

In practice you can always skip the closing p tag. The difference is inconsequential (it only matters for the whitespace).

If I want to convert my work to PDF or MS Word format, that's easy to do if I've used markdown, due to the existence of converters. If the document is written in html, the conversion is more difficult. Any html extensions that need to be converted by hand stand out in a markdown document.