In the Regency period (and for a lot of the Victorian) actors and actresses were viewed as basically prostitiutes. Acting was therefore considered something only very "base" people did and not at all proper behaviour for well brought up young people to emulate. It would have been totally acceptable for them to be playing the piano and singing (which happens a LOT in Austen) but acting was very taboo. People tended to get away with it by doing "tableaus" or playing charades.