While it's true that the CR determination doesn't have to be a 20-step process, it's definitely more involved than looking at a chart.
For example, while Step 4 of Creating a Monster (alignment) will have no impact on the final CR, Step 9, especially the Effective Hit Points part of it - definitely will. And while it's weird that the weapon size damage modifier rules for 5e are in the Creating a Monster section of the DMG, they most certainly exist and absolutely affect the CR calculations. Step 15 is also important but often overlooked, especially the bit about multiple saving throw bonuses, and the Monster Features chart has a bunch of things that impact effective AC and damage output - including guidelines on how to account for area attacks when calculating a creature's damage output.
So the CR calculation isn't a full 20 steps long, but just taking a monster's statblock and comparing its listed AC and damage values to the Monster Statistics by Challenge Rating table generally will not provide a full picture of its CR or why the CR is what it is. The calculation is unfortunately a cumbersome process but it tracks more often than it doesn't if you follow all the steps.