I'm not sure why I'm continuing with this because I don't think all that many arb-el All-Stars are getting dealt but I will.
First let's examine the player.
Arb-el: 4-6 year player
All-Star: Very good player
So, we have a very good player on the good side of 30. Unless, of course, you're going to argue that this All-Star caliber player was kept in the minor until he was 25-26.
Now let's look at the team will to give up a very good, relatively young player.
1. The team has a younger/cheaper player capable of replacing him. Since the team isn't willing to use him, or the younger/cheaper player, at another position, I would think that team is pretty good. After all, they're trading an All-Star. They can't have All-Stars everywhere but, if they do, they're good. Why would I want to help them get younger/cheaper/both?
2. The team is 'rebuilding". Why wouldn't the "rebuilding" team want an All-Star on the good side of 30 for 2-3 seasons in order to be respectable? Because they don't care about being respectable. Why would I want to help a tanker?