Here a pic of a dude, you can see the crease at his waist. Without this extra fabric how would he pull his suit up to get his head in the hole or back out? It would be too tight.
Just imagine if you sewed your sweatshirt to your pants right now and then cut a small slit in the front. When you got to climb out you would either need extra stretch (which we don't have) or more fabric.
Just imagine if you sewed your sweatshirt to your pants right now and then cut a small slit in the front. When you got to climb out you would either need extra stretch (which we don't have) or more fabric.