When you have both braces/keywords and whitespace, it seems to me that you're essentially storing the same information about the program structure in two places, of which one is read by the machine and the other is read by humans. The "re-indent" operation reads the master copy and updates the other one. (And if you forget to re-indent, your human readers will be interpreting an outdated copy and be very confused.)

In Python, on the other hand, the information only exists in one place: the indentation. This is read both my machines and humans. There is no "re-indent" operation as the indentation is the only place the information exists, so there is nothing to sync.

I have heard people bring up programatically generated code before, but as another commenter put it "where Ruby usually denotes code blocks with a `do` and an `end`, Python denotes code blocks with a `:` and an outdent" – so is that really a problem? Isn't it just a slightly different way of encoding the same information?

(As to moving chunks of code in the editor: If the editor has a command to re-indent a selected block, it probably also has a command to shift it left or right.)