Historically and for most people, pies were baked as a way to stretch foodstuffs. "Sugars" were added to make meat and other ingredients that turn sooner more palatable. Lots of baking before refrigeration was done with an eye toward food preservation.

Sugar as a fungible and fiber-free isolated ingredient made dessert baking more economical and of course we love sugar naturally so demand skyrocketed. The Caribbean slave trade and the decades of indentured servitude that followed was really pushed by new high demand for sugars. Pies got sweeter and more decadent. People aren't today baking turtle or rabbit into their pies with a few dried apples for flavor. They're wrapping syrupy fruit in fat and flour -- or in some cases just making pies with sugar fluff alone.

Grandma herself is generations removed from the original rationale for pie and we're even more so. You're right, pie is a blight today despite being oh-so-nostalgic. You don't need to seek out something so obviously sickly sweet as grandma's cherry pie to find issue though. Pasta sauce, bread and even processed meat contain far more sugar than at any time in history. Fruits and vegetables are even sweeter due to our crop selection and our meat contains more fat per gram of protein because of our farming methodology.

Just by eating an ordinary American lunch today, we probably take in in something more nutritionally similar to our former generation's special weekly indulgence than their evening meal; literal pie is just the cherry on top.