Because candidates have lives outside of code. They probably already have a job, so taking a week off for "vacation" which will actually be a "paid interview" sounds like a crummy deal. Writing a patch-- similarly, it's eating into their own time. You're going to lower your hire rate from that.
If they can take half a day or a day out of their life to show up for an interview then certainly they can take the same time out of their life to write some code instead.
but it's paid so you can take a week off between jobs. As I said, give them an option. Interview can take a whole day, doing a project pretty much the same, but you can split it into a few day or do it over the weekend.
