When you get invited to a party, odds are you expect to party. You’ve gotten up the gumption to take a shower, put on a nice outfit, and leave the house. And that takes a lot. So you’d like to be rewarded by a celebratory atmosphere. You want to know that your good efforts were worth something. You want to socialize, to laugh, to dance, to eat some finger foods, and to generally feel a little fancier than you normally do. You put on your good pants, after all. The last thing that you want to do is… manual labor. 

This is like when someone invites you to a «moving party» where it turns out it’s not a party after all but rather a chance for you to help them put all their belongings into boxes. Sure, maybe they provided pizza, but movers get paid good money to do what you’re doing as a favor.