It's pretty much the same here. Often there are HS graduation parties, usually extremely casual open house style, often more than one family sharing a party -- after all, there are only so many evenings. People just sort of make the rounds, often more than in one evening. All the kids just go to all of them!
College and beyond, I rarely see a party. Just a nice dinner out or something. (Maybe included in a multi-occasion party, as we did, come to think of it, when our son graduated college the same time as our daughter graduated high school. But it was really her party; she made all the plans and invited all her friends; he invited one or two of his who happened to be in town. It just seemed a little strange not to include him, if only pro forma, when he was also graduating.)
It seems sort of funny, as we think of university degrees as the greater achievement. Maybe it is because, as someone above said, HS graduation is sort of the launching into adulthood. I think also it is because the families know so many of the other kids in the class, too, many since babyhood. It's almost a community celebration. Later graduations aren't like that.
We never sent announcements for any graduations; maybe they aren't common here. The branch of the family that sent this college graduation brunch invitation sometimes sends HS grad announcements, but this is the first announcement or invitation to a college graduation party we've gotten. It did seem kind of funny to us -- I mean, a brunch to celebrate a college graduation doesn't seem like the kind of thing most people would typically fly thousands of miles for -- but we don't care; we are fond of these cousins.
My own reaction, though, tells me that I wouldn't send them out if I were in their position. I'd send invitations to the brunch only to locals. Any out of town people close enough that they would fly in for a graduation, I'd include in the party, of course, but I wouldn't mention it until I knew they were coming in for the graduation.
And I'm still going to make my husband decide if he wants to send anything! (Let him handle the social obligations for once