RFK Jr and Nicole Shanahan are running, and already on the ballot in 29 states. Though you won't see them interviewed on CNN or Fox News, many polls show them winning against Trump.
One of the reasons that RFK Jr started his campaign last year was a poll by Zogby which "surveyed over 26,000 likely voters across all 50 states, indicated that Kennedy could potentially outperform both President Joe Biden and former President Donald Trump in head-to-head matchups." I believe the poll showed the majority of independent votes going to RFK Jr, and a significant portion of D and R votes going to him as well.
Given recent events, perhaps Trump is now much more popular, but previously polls showed RFK Jr beating Trump.