I see where you’re coming from but “Metal, and some higher quality hard-plastic valve stem caps, often have a rubber washer or seal inside to help make an airtight seal. This type of cap protects from dirt and moisture and also helps prevent air from escaping from a slightly leaking Schrader valve.” (https://www.bluestar.com/get_informed/article/tire-valve-stem-caps-do-your-tires-need-them)
Fair enough. I’ll concede. However, still rather pull that kind of prank with a cover that goes over the original valve.