Nothing opens more easily. The problems start when the text was written on a different kind of machine.
A `.txt` file is characters and line breaks, with nothing describing how any of it should look. That is its virtue: it opens on every system ever made and will still open in thirty years.
Every operating system has an editor that opens it by double-clicking — Notepad, TextEdit, gedit. A browser opens one dropped onto its window. A phone previews it in its files app.
Text written on Windows ends its lines with two characters and text written on Unix with one. Older editors that expect the other convention show the file as a single run-on line, or draw a small box at every break. Any modern editor handles both; Notepad has since 2018.
Nonsense characters where accents or quotation marks should be is an encoding mismatch — the file is UTF-8 and the editor guessed an older single-byte encoding. Most editors let you reopen with an encoding you name.
Treating plain text as Markdown looks like it works until a line beginning with a hyphen becomes a bullet, an asterisk in a sentence turns half a paragraph italic, and a year at the start of a line becomes a numbered list. Converting it properly escapes those characters first, so what the file said is what the page says.