20 isn't that good, especially once you factor in magic items, spells, and fighting styles. Given the monk tops out at 20 unless he wants to use attunement slots on AC items, it makes it even worse at higher level. Not to mention while the fighter or other classes are getting feats to improve combat ability, the monk has to spend them on ASIs to max out his AC. Though I've found buying the Mobility feat is a more powerful defensive ability than a 20 AC.
This rings a little false to me. Anybody can grab magic items, Monk could grab amulet's, rings, or bracers of defense to break twenty, or any number of other things to increase their combat abilities. But honestly, excluding magic items we get Fighters at 21 (Full Plate, Shield, Fighting Style), Clerics at 22 (Full Plate, Shield, Shield of Faith) Paladins at 23 (Full Plate, Shield, Fighting Style, Shield of Faith spell) and Barbarians at 24 (Level 20, max Con and Dex, Shield)... And most other classes chilling between 13 and 19. 20 AC is good, not absolute top tier I guess, but saying 20 is not good just... doesn't make sense.
For Example, to get that Fighting Style means skipping out on Archery, Dueling or Great Weapon Master, sacrificing a lot of potential offense. Casting Shield of Faith costs a spell slot and a concentration slot, and the Barbarian has to use just as many ASI's as the Monk to reach their top tier, and hit level 20.
Besides, Wisdom and Dexterity are also great stats for the Monk, that is all their damage and saving throws, which is something they will want to increase anyways that will also naturally increase their armor. Monks lose very little pursuing that AC, no more or less than any other class topping their stats does, and they have so many combat tricks already that even if they took zero feats they are still quite versatile.