Why the CPF annual limit is not reviewed yearly like the FRS?
The CPF Annual Limit was only quite recently changed, on January 1, 2016. It appears the government doesn't like to change the CPF Annual Limit too often because then employers would have to change their payroll systems' that much more often. Some employers would also see an increase in their labor costs, in the form of greater employer contributions for higher compensated workers.
However, it also appears the government aims for the salary ceiling to include the full wages for about 80% of workers. If and when wages deviate substantially, and a lot more than ~20% of workers are earning above the ceiling, they'll most likely change the limit.
They've also just given us (all of Singapore) a "backdoor" increase of the Annual Limit, on January 1, 2018, with the rise in the Additional Medisave Contribution Scheme (AMCS) limit. The AMCS limit was raised from $1,500 to $2,730 per year. The AMCS limit is above the CPF Annual Limit, and it's also allowed to bust through the Basic Healthcare Sum.