Those events are the face of your group and often the first impression. How's the DJ at your annual dance? Has your carnival become boring? (Hint: Get the fire trucks there for the kids to climb on.) Is the signage attractive? How are your door prizes? You get the idea.
Yes, parents should come because it's the right thing to do, but the most successful groups go way beyond that and make parents want to come to school events. Quality breeds excitement, which breeds parent involvement, which attracts the best volunteers. That circle starts with making the events you currently run that much better.
These articles have lots of suggestions for quality events:
PTO Events That Rock
9 PTO Events Families Will Love