Others may look at it differently, but for me a youngish player choosing to join a team that hasn't won the league in 20 odd years over a team that is nearlt always in the shake up for the CL & PL, who don't have a great manager (just a promising one who learnt everything from the manager of the team that you turned down), where both teams play in the same league and are 200 miles from each other, lacks ambition.
There is absolutely zero guarantee that Arsenal will progress and that last season wasnt a flash in the pan (remember that Liverpool, Chelsea & Spurs all had nightmares), so he may as well stay at West Ham if he isnt going to back himself to be able to be part of a team that guarantees medals (3 times CL semis in 3 years, 5 PL titles in 6).
Plus, Pep has a marvellous track record for improving players (can't say the same for Arteta so far).