Why wrap lines in plain emails? Drupal adds line breaks after a width of 77 in the function wrapMailLine() of the class core/lib/Drupal/Core/Mail/MailFormatHelper.php.

What is the reason for that?

If you open the mail in a small viewport like on a phone, e.g. the line breaks after 50 chars and then again after 27 chars. This behavior is undesired.
Because of RFC2822 (Internet Message Format), section 2.1.1 (Line Length Limits):


  There are two limits that this standard places on the number of
     characters in a line. Each line of characters MUST be no more than
     998 characters, and SHOULD be no more than 78 characters, excluding
     the CRLF.


Why they chose 77 and not 78 for Drupal, I don't know, but I'd bet my house the decision was originally based on that standard.