Although that may in fact be possible in some situations, it is not the way things generally play out.
In manual testing, the test engineer exercises software functionality to determine if the software is behaving in an expected way. This means that the tester must be able to judge what the expected outcome of a test should be, such as expected data outputs, screen messages, changes in the appearance of a User Interface, XML files, database changes, etc. In an automated test, the computer does not have human-like 'judgement' capabilities to determine whether or not a test outcome was correct. This means there must be a mechanism by which the computer can do an automatic comparison between actual and expected results for every automated test scenario and unambiguously make a pass or fail determination. This factor may require a significant change in the entire approach to testing, since in manual testing a human is involved and can:
1. make mental adjustments to expected test results based on variations in the pre-test state of the software system
2. often make on-the-fly adjustments, if needed, to data used in the test
3. make pass/fail judgements about results of each test
4. make quick judgments and adjustments for changes to requirements.
5. make a wide variety of other types of judgements and adjustments as needed.