You're missing the scenario. It isn't that the PCs need 20's to hit, it is the mooks/minions trying to hit the PCs that this happens. It also happens on a round-by-round basis with spells like
shield.
For example, the party is high level and fighting an adult black dragon (CR 14) in its lair. In the final battle, the party's tank (plate +1, shield +1, defense style, ring +1) is AC 24, and the kobold minions are swarming the tank to keep it from approaching their dragon "god". The kobolds will need a natural 20 to hit with their +4 attack bonus. So, if I get lucky and
DO hit, it is automatically a critical hit for double damage. The kobolds can't, RAW, hit in this case for normal damage.
That just irks me.
Another scenario would be an EK in plate and shield with defense style who uses the
shield spell and (for the round anyway) has an AC 26, making even orcs, gnolls, and many other humanoids up to an Ogre, need a 20 to hit. And such a PC is possible by level 3.