Why can't a pop singer on stage, wearing earpieces, hear someone right next to her?
Please see https://youtu.be/P_6i0Js-aw4?t=184. This is Tiffany Young singing Not Barbie, on her Lips on Lips tour in San Francisco, dated March 15 2019.
As her fan (to her right) tries to speak to her, Tiffany appears to say, and I'm reading her lips (pun intended)
I can't hear you.
More detail from Tiffany's MBC Documentary: Good People Documentary Episode 342.
At 8 minutes 50 seconds, you can see Tiffany's earpieces better. But these screen captures are from her Denver, Colorado concert, not the San Francisco concert in the YouTube link. If the English subtitle is correct, Tiffany said "Cause of this, I can't hear very well".