Because they can. And get away with it.
And yet, they continue to lie compulsively, even when the truth would serve them so much better.
The problem with lying on a day to day basis is that somebody (LOTS of people, actually) had recorded statements made in the past, then when the lie is revealed, tend to go back and replay the earlier statement.
Winston Smith was careful, after all, to make sure ALL the records were rigged to reflect the new take on events, and all the “wrong” information was sent down the “memory hole”.
For those of you who missed the reference, Winston Smith was the central figure in the George Orwell novel “1984”.