The only way to play Super Mario Party is with a Joy-Con. Some people already have a set, but if you're looking for a real party, you'll need four Joy-Con altogether. That's why it's no surprise to see people snatching up Joy-Con bundles left and right.
As a matter of fact, the rush to purchase Joy-Cons has caused a bit of a shortage around the world! Various places across Europe, as well as Japan, have all reported low stock for Joy-Con, and sometimes no stock at all! Hopefully Nintendo can remedy the situation soon.