And your point is?
Typewriters don't have a tab "character". The information regarding tab
stops is conveyed out-of-band from the typist to the typewriter, and
doesn't need to persist beyond the time taken to type the document.
Both text editors and typewriters encode information about tab settings
out of band. Editors encode that information in some combination of
program configuration, command-line switches, environment variables, and
embedded mode lines in the document itself. Typewriters encode that
information in the typists' memory, or failing that, in the actual
physical space left on the page. That's a difference that makes no
difference.
My point is that there were well-established semantics for what a tab
should do, and the "8 character tab" is not that. Pressing the tab key on
a keyboard while entering text ought to instruct the editor to advance to
a specified tab stop capable of being set anywhere on the page. Word
processors use that model: the word processor stores the positions of the
tab stops out of band, usually in the "paragraph formatting" or "style
sheet", but in principle they could keep the position of the tab stops
global to the document or even global to the application.
Good text editors also support this model. Some versions of Vim, for
example, include a feature called "variable tabstops". Emacs includes a
variable called tab-stop-list which can set variable tab stops[1]. Even
the Linux command "less" supports variable width tabs, with the -x option.
In case you think this is only for Unix editors, the Windows "Boxer Text
Editor" also supports variable tab stops.
There may, or may not be, good reasons for an eight character default
setting for tab stops. But eight characters is not, and never has been,
the One True Way of setting tab stops.
[1] Although what happens when you press the tab key in Emacs is so
complicated that only three people in the world have ever understood it
fully. The first is Richard Stallman, then second is dead, and the third
has gone mad.