simpleList just adds the simple-list class to the list which is intended to be used in situations where advanced rendering is not required and therefore doesn’t render more complex elements very well. It is not intended to control whether things are on a line or not.
From the OH dacs:
simpleListSimple List
Use for simple lists
From the f7 docs:
Simple List
In case we need list with simple items structure that have only titles, we may use Simple List modification. In this case we need additional class
simple-liston list block with plain<li>items layout:
If the layout of the oh-slider-item widget doesn’t fit exactly the style you’re looking for, the best solution is not to try and change it at the list level, but to create your own custom item widget where you very easily could make sure that the slider is in line with the label and after text (although that gives you very reduced room for the slider, which I suspect was the reasoning behind the format choice in the first place). There are good getting start docs on basic custom widgets here and a very clear How To specifically for default list items here.
That way you can start with a basic list item and add an oh-slider exactly how you want it.