Are there any smartphone applications that will allow you to take macro photos? If not, why aren't there?
I am not familiar with any app of that sort, but it might help you to consider Macro Photography as an optical capability (i.e. phone hardware) rather than a software one.
It all depends on the initial capabilities and characteristics of your phone's camera. Often do mobile phones manufacturers just specify the camera's resolution, that is - how large and dense of an image can be produced from the camera(s) incorporated in the device.
What not many people know is the fact that those camera units on phones have lens characteristics, such as Minimum/Maximum Aperture and Focal Length. If your phone's camera allows you to achieve focus length that is with proximity to the ones on Macro Lenses (most don't), then you may be able to achieve nice results. That includes the maximum aperture your phone's camera can achieve.
If there is such an app, I would say that the only 'cheat' it could do on high-resolution photos is to crop them in a way that the photographed object is magnified and the result still looks fair after loss of resolution due cropping. But that still wouldn't be Macro photography.
After all the physics in a nutshell, you can assume that an add-on to your phones camera may be required in order to alter the focal length of your phone's camera. You may find this question helpful for that matter.