The lower AC of animals is to balance out the HP Bloat druids get, even disregarding Tabletop Balance, in here it provides a balance so that Druid does not completely over take the roles of every class. Druids are tanky, but they are meant to be tanky in a different way from a fighter or Paladin, and even with lower AC because of their MASSIVE possible HP pool, they can out-survive the others without taking in account healing.
Also druids have a lot of spellcasting and other things that differentiate it from fighter, and even if the fighter goes EK, the druid will outclass them in casting, so I think them being a little worse at AC tanking is a fair trade off for increased HP and robust spells.
It is kinda similar to the spell slot difference of Wizards and Warlocks, cause Warlocks get a lot less but get them more often, while wizards get a lot more, but get them less often.
