Why do people give away multiple copies of the same game in separate giveaways?

If they put exactly same giveaway conditions then indeed it doesn't make any sense, but that's up to giveaway creator and his/her preferences. They don't receive any benefits from creating giveaways like this, apart from forcing people interested in all copies to pay price multiplied by number of copies they want to join for.

This might be the only valid reason for doing it like this, but I doubt that anybody actually creates those giveaways for this reason - they simply don't know or don't want to know.

less leeches, force them to enter less by spending more points.

With single copy giveaways, you can easily skim down your giveaway list and see who's marked it and who hasn't. With multiple copy giveaways, it waits until every single winner has marked it before there's any indicator that any of them got it. I like to follow up and make sure the game gets activated, so multi-copy giveaways actually make things a little more of a hassle for me in the end.