They're too young and, frankly, too ignorant to even realize their own talents. They're so focused on trying to earn their cutie marks that they've taken the gifts they were born with for granted. I believe I've seen Scootaloo use her wings very briefly, when she's performing tricks on her scooter. And I can't recall seeing Sweetie Belle use her magic right away... But I could be wrong. It's my greatest hope that a future episode will have them realizing their own unique abilities.