The biggest culprits are food and drink stains, abrasive cleaners, or soap, and water build-ups. Any time you spill a cup of tea or coffee and allow it to dry, it may mar the finish on your quartz countertop. Always be sure to wipe up spills as soon as they occur. Abrasive cleaners, chemicals, and tools can make even the highest quality countertop cloudy.
With regular cleaning using soapy water and a rag, abrasive tools can usually be avoided. Yet, if your tap water is hard, it can cause buildup, or soap suds left behind can cause a cloudy appearance to the quartz. This is why it is important not to let excess water sit on the countertops. Always wipe them dry after cleaning. There are, of course, a handful of other reasons why the countertops may appear cloudy. E.g., they were flawed during the manufacturing or installation process. You placed hot items directly on the countertops or even direct and prolonged exposure to sunlight.