In 51 of the 77 NBA Finals played, the Champion has been one of the two number 1 seeds that year. That is 66.2% of the Finals. And if you take into account the seeds 1, 2 and 3, they total the 97.4% of the Champions. Only 2 titles were won by a team with a 4 seed or higher.
https://www.landofbasketball.com/championships/champions_by_seed.htm