We could in this case but I would definitely not recommend it. There are a few reasons why I did this.

First off, we can now upgrade the package by simply dropping in a new version of the lib without problem. Second we may need to rename some things in 3rd party libs to make the names make sense, multiple libs may, per example have a "base.css" class or use names that does not make it obvious what they belong to. To but everything in the same folder may therefore require us to change 3rd party libs and document each change for future versions. 
Some future libs may also have quirks that does not fit with the folder setup of the theme.

In my experience this is a pattern that can quickly get out of hand. I would recommend keeping the 3rd party setup isolated as the developer intended it. Is is in synonymy with backend code where you keep you own code in source but only reference a "lib" folder or similar for 3rd party stuff.