Dumbledore and Snape didn't just read people's minds all the time.
While they could use Legilimency to find out what everyone is thinking all the time, they don't just go around reading everyone's mind without any reasons to do it. Remember, they're not evil, and they'd certainly be aware that Legilimency is an invasion of privacy. It's unlikely that they make the decision to use it on someone lightly.
They don't usually use it on students, often even when they want information.
For example, Dumbledore and Snape, even if they suspect Harry is lying, usually won't just read his mind to find out. Even though Snape doesn't particularly like Harry, he resorts to yelling at him and threatening him with Veritaserum.
In addition, it's clear they don't just use Legilimency on students when they clearly believe a lie they're told or something else that the student knows to be wrong. For example, it's fairly clear that Snape thinks Harry was the one stealing the ingredients for Polyjuice Potion and gillyweed from his office, neither of which Harry did. Harry did know that Dobby stole the Gillyweed, but he had no idea who had stolen the ingredients for the Polyjuice Potion.
“I haven’t been anywhere near your office!’ said Harry angrily, forgetting his feigned deafness.
‘Don’t lie to me,’ Snape hissed, his fathomless black eyes boring into Harry’s. ‘Boomslang skin. Gillyweed. Both come from my private stores, and I know who stole them.”
They certainly wouldn't use it on a fellow teacher they trust with no reason to suspect anything.
Dumbledore and Snape use Legilimency on students as a last resort. They're certainly not going to go around using Legilimency on other teachers without a reason to do so. For example, Snape might have used it on Quirrell when he and Dumbledore were suspicious of him. However, it very likely that they wouldn't read McGonagall's mind, or even a less beloved teacher like Trelawney. Unless Trelawney was acting extremely odd (well, in a different way than her usual extremely odd behavior) they wouldn't just go reading her thoughts.