In my view, it's a bit more complicated than just convenience or avoiding hassle. The thing is, for young people, home often isn't actually their home. If you're living with your parents or in a dorm, throwing a party for 10-15 people is problematic. The apartment is small or you need to coordinate with neighbors and family. A cafe in this sense is neutral territory where everything's already set up and you won't feel awkward about anything.
But there's another thing too: at a cafe you don't have to play the role of host. You just show up and have fun, and all the service is someone else's problem. At home, as someone rightly pointed out above, you need to keep tabs on the vibe, make sure your guests are comfortable and not bored. That's psychologically draining, especially if there are a lot of people. A restaurant takes that burden of responsibility off your shoulders.
Plus there's the social angle - in a public place, a party looks more "grown-up" and festive. There's a waiter, music, atmosphere. At home, no matter how hard you try, it just ends up being "we gathered for tea in the kitchen." So I think it's not that people are lazy, it's that living conditions are just different than they used to be.