Please realize that, internally in SketchUp, all Layers are equal - there can be no hierarchy. The extensions mentioned by previous replies are tools for organizing the management of layers, which (among other things) include the ability to “group” layers so they can be turned on/off together, and groups can include sub-groups creating what looks like a tree structure. This hierarchy is maintained by the extensions and isn’t noticed by the SketchUp.
Functionally, you’re using a layer hierarchy. And it is convenient, even if all you want to do is change the order of the layers in a flat list.