You must know some pretty shitty people lol. Most people in real life default to telling the truth and we assume most people we interact with on a daily basis are telling the truth as well unless we have reason to believe otherwise. Unless we're saying that everyone of these "normal" people have deep dark secrets that have caused them to mistrust literally everyone they meet, which would be about as far from "normal" as you can get.
People lie a lot to make others feel good, to make themselves feel better, to hide something embarassing that happened to them and these little lies don't make them shitty people. Imagine an old woman who is dying from cancer, but hides her ilness from other people because she is so nice, she doesn't want them to worry. Or a recovering alcoholic who is deeply embarassed by his addiction, he is fighting it, trying to suppress his urges, but in the same time he is making fake excuses not to go to a bar with you without telling about his weakness. Or maybe you are approached by a hobo who reeks of booze, he asks you for some money, telling you they are starving, but you lie that you don't have any money, because you believe that he will buy whiskey instead of bread, after all, you believe you have a good reason not to trust him. People generally tell truth only when it's safe and doesn't endanger them, like most gay people pretend to be heterosexual in countries that punish homosexuality or if there is a strong social stygma against gay people.
Now let's take a look at BG3 companions and why they lie. Gale is one of the companions who asks to travel with you first, he believes that in a group he has a better chance to survive and find healing. It makes total sense for him to omit the fact that he is literally a walking bomb, until he can trust you enough with that kind of information. He is sure that he can keep it under control, he just needs to find some magical artifacts, to keep his bomb dormant, no big deal for him, after all, he's been doing that for some time already, and telling every stranger he plans on spending some time with about it the moment he meet them isn't that smart.
Wyll is reluctant to admit his pact with Mizora, because he is embarassed and feels 'fake' as a true Hero he wants people to think he is. His huge ego is but a mask for his lack of real self confidence, caused by the conflict with his father. He wanted to prove himself, but instead he sold his soul to a devil. He too needs time so he can learn to trust you with his background, because it's extremely uncomfortable to appear vulnerable before people you can't trust.
They lie to you like every normal person would lie about things they consider private, embarassing or sensitive considering that they only lie to you until they begin bonding with you and trusting you after some time.
Most people don't have much to lie about, they also don't usually make appearances in genre fiction. I agree with Alyssa that having a bunch of these neutral to evil companions in an opportunity for some interesting stories to be told. I think the problem people are having is that it can be little much all at once.
True, right now I believe that Lae'Zel and Minsc will be the only straightforward and honest companions you will get. Maybe Karlach too.
