Furniture Joint

Had a dining chair fail on normal use. Less than 2 years old. I can't get my head round this joint (photos of both parts). How did the staples get there. What do they do? Was it bound to fail? Greatful for advice.

They were driven in to hold the joint while the glue sets. As you can see, the profile of the tongue here is probably too small to support the load [unless you've abused it!]. Even if it was big enough, you would struggle to glue this back together. I would put a couple of large screws into piloted holes to repair this [missing the bolt that passes through the leg].

i fully agree with "why not indeed"
buuuuuttt because the side you are screwing into may be around 8-12mm before reaching the pocket [mortice]before hitting the tennon i would washer the screws as well