Why is there no notion of raising in Liar's Poker. Do some variants have it?
Probably because it isn't really a vying game in the sense that nobody has a hand that they believe is stronger than their opponent's. Instead, they have a communal hand and they are betting on their guesses as to what it is.
(The bridge bid of 'double' (and even of doubling for take-out) might make sense as a liar's poker variant.)
Liar's Poker: A Winning Strategy