[SAI_Peregrinus]

If that were the case tabs would be fixed width too. But they're not. There's no fundamental reason tabs have to be the only variable-width character.

[eikenberry]

Tabs would only be variable in the number of spaces uses, but those spaces will be fixed width.

[jononor]

And spaces could be variable in the number of "sub-spaces" used. Each could be say 1/8 of a regular space.