The answer to 'why is my bathtub constantly running' is that the most common cause is a problem with the bathtub's drain stopper or overflow mechanism. This can lead to water continuously flowing down the drain, even when the tub is not in use.
Some common symptoms of a constantly running bathtub include the tub never fully draining, water continuously dripping or running, and a higher than normal water bill. The good news is that many of these issues can often be fixed with a simple DIY repair. However, if the problem persists or you're not comfortable doing the repair yourself, it's best to call in a professional plumber to diagnose and fix the issue.