Because mempool.space identifies them for you

It's indicated in the details on mempool.space

The best questions reveal themselves when someone asks for clarification. 'How do you know it's a runestone if you don't know what it is?' That's a person who's used to being told what to think and decided to push back.

Define 'runestone.' If you can identify something without defining it, you're recognizing by pattern matching, not understanding. That works until it doesn't.