Live poker requires more focus....there is more to pay attention to....watch your opponents when they are in a hand, and when they are just watching play. Notice how they play with their chips, chew their gum, how they look when they are strong and weak....the key is to store it all in your hard drive (brain) for use at another time...sometimes you will get only one good chance at using a tell against an opponent in a big hand, and that's when it is most important.
Online, I find that I am listening to music, watching YouTube, or just playing multiple
poker games...all of which are not optimal for poker success.
Lastly, I agree with those who say that players are far less likely to do really stupid plays live, because everyone stares at you and wonders how the hell you could have just done that!