This is a good place to wonder what goes on with Apple’s engineering teams. Joe Rossignol:
The iMac Pro was released in December 2017, but standard 21.5-inch and 27-inch iMac models have not been updated in over a year and a half. Since the original iMac launched in 1998, the computer has typically been updated at least once per year, with the sole exceptions being 2016 and 2018.
In other words, not often. iPhones get upgraded every year.
The iMac is not alone. The 12-inch MacBook and Mac Pro have also set record-long spans of 602 days and 1,866 days and counting respectively since their last refreshes.
602 days just seems too long.