It's true that Arceus currently has the highest total base stat total of all Pokemon, excluding mega evolutions. So why isn't it here? Well, first, there's its lackluster ability that can shift its type to whatever type of Plate its holding. This lets you alter Arceus's element, sure, but doing so uses both an ability and a held item slot that could have been used for better purposes.
Then, Arceus's base type (Normal) isn't particularly good defensively, and Arceus doesn't learn many important moves while leveling, although its broad access to TMs helps there. Overall, it's definitely strong, but a different legendary with a useful ability and cleverly held item could probably take an Arceus.