He apparently found no better offers.
Adrian Wojnarowski of ESPN:
This will be Carter’s 22nd season – most in NBA history. He’ll break a tie with Dirk Nowitzki, Kevin Garnett, Kevin Willis and Robert Parish. If he plays on or past Jan. 26, Carter would also become the first 43-year-old to play in the NBA since Willis in 2007.
Carter’s longevity is incredible. I wrote about it four years ago, and he’s still going!
An amazing athlete in his prime, Carter has remained in excellent shape. He has transitioned into a stretch four late in his career. He’s strong enough to defend opposing bigs, and his outside shooting/mobility are positives at power forward.
But if that if that young group is ahead of schedule, Carter could help Atlanta compete for the playoffs next season. If it takes a little longer, Carter can provide veteran mentorship in the meantime.