Purely social events

Because if you aren't doing it for a charity (and honestly a lot of these things spend as much or more on the event than they end up actually sending to the charity), you will get a bunch of 99%er protestors staking out your red carpet

Why would the media want to cover/photograph/blog random parties? They no doubt happen but I can promise you it's not being reported on. Also, like OTC suggested, people feel better about dressing up for charity events/awards/etc (aka a purpose) and not random events.

Such essentially social events still occur (just as fundraisers occurred back in the day), but a fundraiser for, say, cancer research might involve 600 guests (some of them celebrities, of various status), in a public venue, with P.R. people representing both the cause and the celebrities actively courting media attention.

Whereas a modern day Astor or Vanderbilt, inviting some friends/acquaintances/associates over to his duplex at Park and 53rd, for a dinner party, is something entirely different. It's a private setting, involving few people, quite likely none of them pop stars, professional athletes, or other celebrities of a particularly trendy sort. There are no P.R. people working the event. And, indeed, in certain circles a host with a reputation for courting attention for such get togethers may well acquire a negative reputation amongst his potential guests, since publicity and the public spotlight is decidedly not something his guests value, as a general rule. Courting media attention is regarded by many as vulgar. Certainly, exceptions may sometimes be made for a worthy cause, but not for a private dinner party.

I hope this helps explain why you may hear lots about the former sort of event, but relatively little about the latter sort of event.