why are the animals so easily fooled,even when they find squealer with a ladder and white paint beside the barn at night?

The animals naively believe that the pigs have their best interests at heart. The other animals are ignorant and fairly illiterate. They believe that whatever Squealer is doing is in line with what Old Major believed. They do not have the sophistication to interpret propaganda.

Hey, Callie! That's a great question.

This guy just explained the EFFECTS of the animals' stupidity, but you asked about the CAUSES of it. So:

The reason the animals are so stupid is because Napoleon has pulled the wool over their eyes (pun intended, sorry). The reason they are so easily fooled is (mostly) the most simple answer: They are just plain dumb. And the book describes them this way. Boxer can't write more than ABCD and the only animals that can read (besides the pigs) is Clover the horse and Benjamin the donkey. Now that is something interesting, because whenever Squealer wants to prove a point, he'll say that documents have been recently discovered which prove .... (whatever it is he's saying) and he'd show the animals the documents, but they wouldn't be able to read them, and so on. And when Squealer is found with the paint at night, not all the animals are fooled. Benjamin understood, but refused to explain. The reason for this is that the idea of Napoleon being a wonderful leader has been so deeply ingrained into their heads that anything that says otherwise simply will not be considered.