That may be true for codebases that always use a single tab for each indentation level -- I get that. But to stay readable regardless of tab width, it requires a coding style that keeps indentation strictly consistent with syntactic context (not an issue in Python, since indentation is syntactically significant already).

It also limits what developers can do to align things in JavaScript, Lisp, Ruby, C#, etc. -- especially when using complex call chains/lambdas, large literal data objects, s-expressions, long parallel repetitions blocks of code, etc. In those cases, using spaces is a huge benefit, because it provides a way to scoot things left or right to make them more readable. With tabs, you can't do that unless you mix tabs and spaces -- which is not cool at all.