Then you are building your character wrong. Wizards and Warlocks can get quite decent AC.
Firstly: Dexterity isn't a dump stat for any class other than heavy armor wearers. Always tro to aim for at least 14 in dexterity.
Secondly: Constitution is a main stat for any caster, because it affects your concentration checks. Wizards have low HP to start with, so having 14 or even 16 in constitution is very smart.
Wizards has access to mage armor, ALWAYS use it. With 14 in Dex you'll have and AC of 15, which isn't bad.
Warlocks has medium armor, so you should be starting with and AC of 14 if you tool 14 Dex. From here you can actually make a choice, go for 16 Dex at character creation, to start with AC 15, or you can pick up medium armor proficiency at level 4. (If we get variant human you can do this at level 1)
Or you can be a class that starts with medium armor, like Gith
There are also a plethora of other defensive spells, like mirror image and blurr that can help you out. If Larian get's a good working reaction system then Shield is another very good defensive spell.
As for most low AC characters: Keep them back, and hope they don't get shot.
There's an issue with BG3 at the moment though, it seems the game is programmed to target low AC characters or something, which means your caster will get hit more.
