So? A lot of performers do it. Bruce Springsteen uses one. Tom Petty did. Paul McCartney does. As does Elton John. All of ‘em use teleprompters. It’s called being prepared.
Here’s Aerosmith using one.
It’s not that they didn’t take time to learn the lyrics. Sometimes you can’t remember them all, especially when you have a large catalogue of music like some of those I mentioned.
I once watched Billy Joel live and he flubbed a lyric near the start of a song. He stopped, laughed it off, and restarted the song laying emphasis on the lyric he messed up.
Memorizing lyrics for a song you’ve written and need to perform in front of people and keep separate from other lyrics you’ve written is the sort of difficult thing that seems pretty simple right up until you actually try to do it.
I wish I knew what exactly it’s from, but there’s a recording of Tori Amos live where the line she’s trying to sing is