Saito knows an excellent employee when he sees one.
Certainly Mr. Cobb is the best at what he does-but that isn't the same thing. Mr. Cobb is unpredictable, impossible to trust. But Arthur is meticulous, immaculate in his process. He has the qualifications to be Saito's personal assistant on his precision with the briefcase contraption alone. His stable presence is reassuring to Saito, who could use a companion with some social grace, with some patience.
The slim cut of Arthur's suits doesn't hurt, either.