I'd have to guess this wasn't as necessary as the restraints are designed in such a way that guests can't really make the motions of getting off the ride until their restraint unlocks.
Granted, in reality all properly secured restraints should prevent guests from prematurely getting off the ride, however in rides with AC's clamshell restraints, there's a lot of freedom of movement whereas on Alpie the most guests can do is unbuckle a seatbelt (that doesn't really contribute anything to securing the restraint) and swing their legs in the air.
Though Griffon has similar OTSRs as Alpie, I think the lack of the train chassis over guests heads gives a larger feeling of freedom necessitating a spiel to keep guests from attempting to prematurely exit the ride.
Granted, in reality all properly secured restraints should prevent guests from prematurely getting off the ride, however in rides with AC's clamshell restraints, there's a lot of freedom of movement whereas on Alpie the most guests can do is unbuckle a seatbelt (that doesn't really contribute anything to securing the restraint) and swing their legs in the air.
Though Griffon has similar OTSRs as Alpie, I think the lack of the train chassis over guests heads gives a larger feeling of freedom necessitating a spiel to keep guests from attempting to prematurely exit the ride.