All that stuff is nice, but realistically it's not always that way. Sometimes you need and want discovery (it can help the parties better understand one another). Also, the arbitrators work for profit: you don't want to send people away bitterly disappointed, because they'll badmouth you out on the street and cost you future business. After all, when you rule against someone, it's not like they think you were right. So the arbitrators are under pressure not to antagonize the loser, so they tend towards compromise decisions, rather than just siding with the side that's correct (if there is one). That's not always a problem (union labor negotiations, for example), but it can be, if you're entitled to something and you believe you're right.
The absence of procedural/evidentiary constraints enables outrageous behavior (manufactured evidence, even).
And in complex situations, it's not necessarily that much faster than trial.