Not all players on defense take the right angles to get the tackle. Passes like this work very well with a speedster wr. If this play never worked well then offenses wouldn't have been using it for decades. Alot of offenses plays if not all have been done before what makes them great is timing and the players.Throwing horizontally allows defense more time. Always has always will.
So why throw sideline screens? Gets those big boys running sideline to sideline and combined with the up-tempo wears the defense out quickly. And like CRG says you have to break the big guys before they become completely gassed or they are done for the game.