So, thinking they are automatically lying is even worse than thinking they must be telling the truth. The position you need to hold is between the two.

The reason to hold it consistently is to take advantage of habit building and using how your brain works to your own advantage. You can try to calculate an independent “likelihood” for every claim if you want, but you’ll frequently be wrong, just because you can’t take everything into account. And it’s a massive waste of energy.

As to why, it varies. Humans are very different from each other, so the reasons will be many and varied. But the important thing to remember is just how easy the lie is, and how there’s really no consequences if someone does.