Many games now offer a trial period, which is the same as a demo.
Also, back then the only way you could ever see a game was to play it, or have a friend with it. Now you can watch lets-play videos, watch video reviews, etc. It is WAY easier to see what all is in a game and how it plays without actually playing it.
Personally, if the game doesnt have a trial period, and its not some game I have been waiting for (Like the Witcher 3 for instance), I go and watch lets play videos (via YouTube or Twitch). There are some really good people out there that do a great job at making them.
So in short, there just isnt a need for most big games to have them.