This is because of a decision made originally to keep things simple, which is the overall purpose of the builder. Allowing more options = allowing more to go wrong.
It is something that we feel needs improving however, so we are looking at making changes.
In the meantime, you can manipulate the source code a bit if you feel up to it - starting point, adding a side nav and then using display: none and changing width on content area.
Note we do not suggest this as such, and can not support it if you decide to go ahead, so I strongly advise otherwise unless you know what you are doing.