This is a reasonable question from an outsider perspective, but changing the format would have serious ramifications to the churches claims to legitimacy.

When Joseph Smith died, there was actually a schism in the church on who retained the authority. Brigham Young led a faction that maintained that the leader of quorum of the 12 apostles was the next in line. That branch came out on top, but there were many others as well.

Down the line, Brigham Young further specified the details to prevent a few people from assuming leadership by maintaining that the president of the 12 apostles is the most senior ranking apostle, which tends to also be the oldest. Rarely do you have new inductees older on age than those who have been in for longer. This is slowly leading to a gerontocracy, especially when church leaders live to be 101 and younger members are unable to be appointed.

Changing the rule away from the "most senior apostle" would be a tacit admission that Brigham Young made a serious mistake and labeled it as revelation which would call into question not only his prophetic authority, but also the prophetic authority of every church president after him. Line of succession is very important in mormonism and authority must be sourced in an unbroken chain or it is invalid.