This news being "embargoed" until a conference is baloney, all it does is enable insider trading. We have this thing called the Internet that you are looking at right now that does a far superior job of mass dissemination of information than an archaic conference. Ideally clinical trials should release their information in as close of fashion to real-time as possible, the conference system is obsolete and needs to go away...
I suspect you may be one of those people who texts friends that are sitting beside you rather than open your mouth and speak. Some people still are capable of doing business and socializing in person. As other posters have noted, conferences are held in cities that offer some fun and games. Don't forget it's all a business expense.
you are only looking at it from an investors or stock traders point of view.
set conference dates allow the company to take care of the business aspect of the business.
and make internal corporate decisions and comply with the not insubstantial govt. regulations regarding announcements
Conferences double as party time for those who attend. Have you noticed that they are often in party towns like Los Vegas and in the case of this years ASH, New Orleans? I remember Profs. at Universities going to such events at locations like Cancun, Mexico.