to the person above me, red hot chili peppers still bring tons of energy to the stage and have like tons of dedicated fans, so it makes sense they're still headlining imo
There's multiple reasons why RHCP is still headlining:
1. John Frusciante is back in the band.
2. The tickets prices for their regular shows are extremely overpriced. If you missed the chance to see them there, you can see them at this festival instead.
3. Even though you are right about their recent work the past 15 years, they still did make "Under The Bridge" as you mentioned. I mean, just getting to see that song would make it worth it for some and they are great performers.