yeah in an ideal world, you are right. Having been at a couple centers and a large Tracon, most people should be able to get through every sector in 2 years of just training time if they have it.
However, since we don’t have that ideal situation due to staffing, backlogs, vacations, technology upgrades, and human performance, it’s not the norm.
Some people are sharp and hit it right, and it works.
Maybe the Next person shows up, and then gets delayed by a bunch of knuckleheads in front of them. Or maybe they are the slow one and delay the people behind them and struggle, having a review board or two, some article 67 time and maybe some skill enhancement training too just to slow it down further.
Then for fun, add in some CPDLC, ERAM or other upgrade and bam, add a year. Maybe more.