I would say that coaches and AD's permit, or even encourage, dancers to gloss over the mime because they think (or they think the audience thinks) that it's boring, but that it has to be in there because "Giselle" is the oldest continuously performed ballet and it's their duty to leave it unchanged.
That, or--since they know what all the gestures mean--they don't notice that they aren't being performed very clearly since they automatically "fill in the gaps" mentally.
