1. Direct Interaction with the Browser
When you launch Selenium on your own PC, the open browser is the same one you normally use. If you move, resize, or minimize it, Selenium receives unexpected signals. These actions change the window's state and can disrupt the script's execution, as Selenium waits for the browser to remain stable before running its commands.
2. Conflict Between Human Use and Automation
Your computer tries to manage both your manual actions and those of the automated script. For example, if you minimize the window, Selenium may no longer "see" certain elements or lose focus on the page. This skews the results because the test is no longer run under the intended conditions.
3. Advantage of a Cloud Environment
In a cloud solution (EC2 with virtual desktop, WorkSpaces, BrowserStack, etc.), the browser runs on an isolated machine. No one touches the window while Selenium is working. The result: the tests are consistent, reliable, and independent of your physical interaction with the PC. This is why cloud environments are better suited for automated testing.