Why unit tests should not use database?

Question

Reading an article Evil Unit testing, I am not exactly sure WHY the unit tests should never use DB, network or file systems. What if it an network app?

Answer 1

Unit testing is not intended to test the complete functionality of an application. It is meant to make sure that definable modules of code work as expected. To test an application it is not enough to use unit tests. You must also perform functional testing and regression testing. Database access falls outside the scope of unit testing, so you would not write unit tests that include database access. You would include database access testing in your functional tests. Similarly, if you have a network app, you would not include network access in your unit tests. In both cases you would unit test the code that manipulates the data that it gets from the db or the network using simulated-known data and leave the actual access out of it.

To reiterate, it is not enough to just unit test an app.