sphere's...plus a demonstrated ability to produce positive results, along with a history that clearly shows where their priorities are. In the case of JB, the public knew he is reaching his "Use By Date", but valued those attributes...and his record thus far has justified their votes.
JB's run in 2020 was precipitated by the threat of DJT getting another term...looking ahead to 2024, that threat has not gone away...and if it had...i.e. had Mitch McConnell and the GOP stood up to him and convicted him when impeached...IMO Biden would have graciously stepped aside and let 'New Blood' take the stage.
Now, as to why GOP voters cling to a man of equal age, but much poorer record...I have no idea.