A second, and equally important reason, is that they are super easy to share. Everything the person needs to know in order to make the decision to retweet your quote is right there in front of them.
If you tweet blog posts, people tend to stop short. If you haven't read the post, and don't know it's quality, you may not want to risk sharing something that could be embarrassing or at a minimum, simply not interesting.
Thus, quotes make a great way to create a shorthand connection with your audience. Pick them carefully and take the time to create some graphics using canva.com or quotescover.com. Good graphics generate more engagement.
You can also download jukeboxes of picture quotes for free from tweetjukebox.com, my automated tweeting site. Check it out.