>It is completely arbitrary and we have arbitrarily converged on a diamond as a mechanism to do this.
This isn't quite true.
1) Who's "we"? Outside the US, no one really cares that much about diamonds. Outside the US, western Europe, and Japan, no one does. It's largely an American phenomenon.
2) People didn't just "arbitrarily coverge" on diamonds; this was engineered by DeBeers almost a century ago, in what's probably the most successful marketing campaign in human history.
>If we are talking about financial commitment and signaling to a partner, why not gift a house, a car, a long term bond, cash, expensive clothing, a watch, literally any other piece of jewelry, etc?
This is literally what's done in other cultures, and has been for ages. Though usually, it's the bride's family who has to pony up all this largesse, in what's called a "dowry".
>Why does it have to be the guy that does it; shouldn't the wife also signal equal levels of commitment or is she incapable of that?
This is a very good question, because again, in other cultures, it has traditionally been the bride (or rather, her family) who had to provide a dowry to get a decent man to marry her. This of course has, in more recent times, driven families to abort their unborn female children in favor of male children.