The biggest reason people say they don't volunteer is that they just don't have the time. I used to buy that, but people have watched cat videos 25 billion times. (True). So, let's not assume that they don't have the time. They might not want to spend time volunteering, but they have it.
And if they did spend some time helping, they might feel that they actually have more of it. Wharton's Cassie Mogilner has conducted a few studies published in the Harvard Business Review that show giving time to help others makes you feel like you have more time — that you are "time affluent".
Obviously giving ALL of your time to others won't give you that feeling, but finding the right balance will make you feel, and probably be, more productive.
I don't think the cat videos are doing that.