His impact on the game is probably similar to that of a celebrity in an ad: the celebrity isn't cast because of their acting/posing ability, they're just very likeable, recognizable people that you want to associate your product with. The electric reception he received at the game announcement show and the reddit memes he has generated seem to prove this. No one was under any illusions that Keanu was going to elevate the story, they just really love Keanu.

He's also, bluntly, not a very good actor

He's a great person. His acting range is limited, but within that range, he's good at what he does. Whether that makes him a suitable actor for a computer game is debatable, but on the other hand, he's also a beloved actor, so it's probably good PR.

I have no problem with them including Keanu in the game. I do have a problem with crunch and releasing a very buggy game.

I take Tom Hanks or Bill Murray over Keanu any time, but he's extremely dedicated. The amount of work he put into gun and martial arts training...Maybe that's part of being a good actor, after all.

His lines in this game... Sometimes I'm really just cringing at how demotivated/drunk/high they either want him to sound or he sounds by default.

some actors are just pretty faces. Orlando Bloom also springs to mind.

You make it sound like having a leading role in a game is the highest honor in the land one can bestow upon an individual.

Or Maradona