Selecting a number of layers from ArcCatalog and adding them to a map by dragging and dropping them onto the ArcMap Table of Contents (TOC) will not order the layers based on feature class. For example, polygon layers will not automatically reorder to the bottom with arc layers in the middle and point layers on top; instead, the layers will be arranged in the TOC by the order they were selected: the layer selected first will be at the bottom of the TOC, and the layer selected last will be at the top.
ArcMap allows you to place layers in the TOC where you want them. If you already have layers in your map, you can drag new layers and drop them anywhere in the TOC. Automatic ordering of layers by feature class would inhibit this functionality.
If you wish to have the layers ordered, in the TOC, by feature class, you can drag and drop the layers onto the data frame instead of the TOC. Dropping the layers onto the data frame will order the layers by feature class.