First of all I would like to remind you of our community guidelines for these forums https://twittercommunity.com/faq - thanks 
We’ve explained how this works via several threads here in the past, apologies if you felt that we didn’t address your question adequately before. Having said that, the behaviour has changed since the original poster added this thread in 2014.
Today, the way things work is as follows:
- Cards are expanded by default in the timeline on the Twitter apps for iOS and Android, as well as in Tweet detail view.
- on the Web, you need to click the View Summary link to see the Card content.
We constantly look at ways to improve the user experience and that is one reason why you’ll see the Cards expanded by default on mobile now, when they were not before.