I assume I'm overlooking some subtle (maybe obvious) design principal of SharePoint 2013. 

The default permisions for site Members is edit.  This gives that group permission to create pages, lists, librarys, etc.  For a site with many members that might lead to a lot of unmanaged, uncoordinated changes.  I would think Edit rights are for a smaller group of trained individuals and Contribute rights for the masses.

Why is there no default group with Contribute rights only? 

Possibly in cases as I described it is up to the SharePoint administrator to add that additional group with Contribute rights, so they can keep the Members group with Edit rights down to a relatively few trained individuals.

Hi,

You can create custom permissions if you need

https://technet.microsoft.com/en-us/library/cc263239.aspx