Indeed, that’s the most baffling bit about this entire business. And it was Kasdan who apparently pushed hardest for their hiring in the first place.
The other influence on the film’s intended style was a “heist caper” type of film. I can understand thinking Lord & Miller might work for an Ocean’s Eleven style movie. But if you want to mix that sort of lighthearted film with the grimness of something like For a Few Dollars More? That’s a different order.