This is a classic problem of externalities. You can only quantifiably judge what you are quantifiably measuring.
In this statement, you are only measuring the file couplings. I hope it's obvious that there are many other factors at play.
For example, let's look at a "typical" development iteration for a feature.
1. you write the automated test
2. run the test
3. if the test passes goto 6
4. edit the software
5. goto 2
With manual testing:
1. edit the software
2. manually test the software
3. if the software does not do what you need it to do, goto 1
TDD gives you a quick feedback loop, since the verification step is automated, at the cost of up front time spent on writing the test.
There's also automated regression testing, which is useful to prevent regressions when you change the software system.
When the software becomes complex:
* regression tests offer a quick feedback loop that scales almost linearly
* manual tests have a slower feedback loop and are often given to the QA staff, which involves communication overhead, scheduling, meetings, etc.